Here at WWM we frequently get messages from aquarists who
aren’t just dealing with one sick fish but the total failure of their aquarium
to run properly. There are a bunch of reasons for why this should be, and
identifying the exact cause of your woes can take time. That being the case,
here’s some general advice to keep your fish from getting sick or dying before
the underlying problem with the tank is solved:
(1) Water changes dilute problems! The important thing is to
ensure that the new water has similar water chemistry and temperature to the old
water. It also needs to have water conditioner added, plus any other supplements
(such as salt or pH buffer) that you normally use. Changing as much as half the
water per day is not unreasonable when dealing with an unstable or otherwise
(2) Don’t feed your fish. There’s no need, your fish being
perfectly able to go for weeks without food. Furthermore, putting food in the
tank will cause water quality to drop, which you definitely don’t want to
(3) Check the basic mechanics of the tank are working. Make
sure the filter is running, and that any attachments that help oxygenate the
water (such as spray bars) are connected. Is the water at the right temperature?
Check the heater is plugged in and set to the right temperature. Turn the
control dial slightly to see if it switches on and off properly, and then turn
it back to the original setting (for most tanks, 25˚C/77˚F is about right).
But having got the tank stabilised as best you can,
it’s time to troubleshoot it…
When more than one fish gets sick in a short space of time, the most
likely cause of trouble is water quality. There’s a three-step run-down
here that should shake out any possible issues:
Firstly, is the tank overstocked? The old ‘inch per gallon’ rule is a
good one, in the sense that’s conservative and allows a bit of space for
fish to grow, breed or even for the hobbyist to add a few more fish once
the filter is properly matured and running well. It works very well with
small fish (Neons, Guppies, and so on) when stocked in tanks 8-10
gallons in size or larger. It doesn’t make sense with anything larger
than an Angelfish though. If you’re keeping something like an Oscar,
then it’s adult length of around 15 inches definitely does not mean you
could keep one in a 15-gallon tank! It’s not a good rule for very small
tanks either, anything smaller than 8 gallons not being suitable for
ordinary community fishkeeping.
An alternative, though less conservative, rule is to allow 10-12 square
inches at the surface of the tank for every inch of livestock. A
standard American ‘deep’ 20-gallon tank measures about 24 by 12 by 16
inches, so the surface area is 24 x 12 inches, or 288 square inches.
Divide this by 10 and you get 28.8, which is the number of inches of
fish you could safely keep in there. That could be fourteen 2-inch
Platies, eleven 2.5-inch Corydoras catfish, or some multiple thereof.
Because it’s less conservative than the inch-per-gallon rule it doesn’t
have the same margin of error, so if you’re using this rule, you should
factor in growth and breeding before you fully stock the tank. Also,
bear in mind this is another rule that applies to small fish, certainly
nothing bigger than an Angel or Gourami.
What happens when a tank is overstocked? As always, reality trumps
wishful thinking, and an overstocked tank will see fish dying off until
a ‘sensible’ level is attained, at which case things will seem to settle
down. Over time those fish will grow or breed, in which case there’ll be
another spate of sickness and death. Obviously it’s much better to stock
the tank sensibly from the start, not least of all because you’ll save
yourself the expense of medicating sick fish or buying replacements a
few weeks down the line.
Secondly, is the filter big enough? Aquarists rate filters in two ways,
the amount of filter media and the ‘turnover’ rate at which water is
pumped through the filter. Most aquarium filters will provide
information on these, and there are some rules of thumb that can be
handy when choosing a filter. For the average community tank for
example, you want a turnover rate of around six times the volume of the
tank per hour. In other words, if you’ve got a 20-gallon tank, you want
a filter rated at 120 gallons per hour. But to keep things simple, most
filters will state the aquarium size they’re designed for on the
packaging. These tend to be on the optimistic size, so if you’ve got a
well-stocked tank, or you’re keeping fish bigger and messier than Neons
and Guppies, then the default filter might be overwhelmed. Either add a
second filter, or swap the existing filter for one that’s a bit bigger
(carrying across as much mature filter media into the new one as you
How do you know if your filter is too small? If your ammonia or nitrite
test kit register anything above zero, but your tap water doesn’t
contain them, then ammonia and nitrite are accumulating in the aquarium
more quickly than the filter can remove them.
Finally, has the filter being properly matured and maintained?
Biological filters will take around 6 weeks to mature, sometimes less,
but even with the addition of filter aids and bacterial cultures,
‘cycling’ a tank will take a few weeks. Before the filter is mature
ammonia and nitrite will rise above zero, and these are toxic to your
fish, either poisoning them directly or, at lower levels, stressing them
enough to make them prone to diseases such as Finrot. As mentioned
earlier, frequent water changes will dilute the ammonia and nitrite.
Obviously, don’t add either fish food or new aquarium fish until ammonia
and nitrite are back to zero.
Adding ammonia remover (also called zeolite) isn’t a solution here.
Chemically removing ammonia will slow down the rate at which filter
bacteria grow, and after a few days the zeolite will be saturated
anyway, in which case you’re back to square one. What you need to ensure
is that the filter has plenty of biological media and that media is
properly looked after. Unless you have a specific need to use carbon
(and few aquarists do) remove this from the filter and replace it with a
good biological filter medium, such as ceramic noodles or sponges. Rinse
these out periodically, perhaps monthly, but not under hot water!
Ideally, clean them in buckets of aquarium water, or else lukewarm tap
Water chemistry problems
Fish are adapted to a specific set of water chemistry conditions in the
wild, and while most have some degree of flexibility, too much change
can stress or kill them. So far as the casual fishkeeper is concerned,
there are three ways water chemistry might cause problems:
Are you trying to keep fish in completely the wrong conditions? If you
try to keep Mollies (and most other livebearers) in soft water you’ll
probably find that they won’t stay healthy for long. Conversely, species
adapted to soft water, such as Neons, tend to be disease-prone when
maintained in very hard water. Then there are the brackish water species
like Scats and Monos that need some salt added to their water; without
it they’ll get sick.
Test your water chemistry (or have your retailer do it for you) and
choose fish that are known to tolerate or thrive in those conditions.
There are numerous books and websites that disclose the water chemistry
tolerances of particular fish, including this one.
Is something in aquarium changing the pH or hardness? Most aquaria tend
to become acidic over time, but this is normally a slow process easily
offset by doing water changes every week or two. One or two bits of
bogwood shouldn’t have much effect, but very large amounts of bogwood
relative to the size of the tank can soften and acidify the water much
more quickly, which may stress fish unsuited to such conditions. The use
of a pH buffer may be necessary if you have soft water to ensure the pH
doesn’t become too acidic. Your retailer will have a range of these
products on sale, often described as Discus buffer because it’s most
often used in tanks with Discus.
Similarly, the addition of calcareous materials such as limestone rocks
or large seashells can have the opposite effect, raising the pH and
hardness over time. Again, water changes will minimise this, but unless
your fish prefer hard water conditions, it’s best to leave such
materials out of the tank.
Is your tap water chemically unstable? Some tap water contains a lot of
dissolved carbon dioxide, which lowers the pH. This is most common where
the water comes from an underground aquifer or deep well. The chemical
processes involved aren’t of importance here; what matters is that once
the water gets into a bucket, that carbon dioxide will start to diffuse
out into the air, especially if the water is warmed up.
How do you know if you have high levels of carbon dioxide in your tap
water? Try drawing a bucketful of water tonight, do a pH test on it,
leave the bucket somewhere it won’t get spilled, and then do another pH
test the following evening. If the pH has risen significantly, then
dissolved carbon dioxide leaving the water overnight could be the
Very soft water is another possible reason for unstable water chemistry.
What you want here is a carbonate test kit; anything much below 3˚KH is
likely to have very little innate buffering capacity, which means that
the tendency of aquaria to become more acidic over time could be
exaggerated. If you need soft water conditions for the fish you’ve
chosen, then your retailer will have pH-stabilising Discus buffers that
will help to fix the pH between water changes. Otherwise, the use of
one-half to one teaspoon of sodium bicarbonate per 10 gallons will raise
the carbonate hardness sufficiently for a general mixed community tank.
What you’re aiming for is carbonate hardness around 5-10˚KH, the
lower-end of the range for things like tetras and danios, and the
higher-end for livebearers and rainbowfish.
Water temperature problems
Most of the fish we keep in tropical community tanks are happiest around
25˚C/77˚F, which will be a bit warmer than most homes. For this reason a
heater is usually required. If the fish are too cold they will be
sluggish, disinterest in food, and eventually become sickly and die.
Fish that are kept too hot will usually gasp at the surface first,
desperate for oxygen as the water warms up, but sooner or later expire
from heat exhaustion. It should also be remembered that adding water to
the tank that is too hot or too cold will also stress, even kill, your
Do you have a heater at all? Contrary to what some people think,
tropical fish can’t be kept at room temperature or under an angle-poise
lamp. There are coldwater and subtropical fish out there that may be
kept in unheated tanks, but your standard community fish, including
Bettas, will need a heater. Your retailer will have a selection in
stock, the main thing being to choose one appropriate in size (wattage)
to the volume of your aquarium and the coldness of the room. Don’t
choose a heater that’ll have to work flat out to keep the tank warm:
such heaters will be much more likely to fail. So if you have a
20-gallon tank, don’t choose a heater rated for tanks 10-20 gallons in
size, but instead go for the 20-30 gallon option instead. Similarly, if
the tank is in a cold basement, get the next size up, because the heater
will need a bit more power to overcome the chilling effect of the room.
Is the heater at the right temperature? While aquarium heaters normally
have a built-in thermostat with a dial for setting the temperature, the
numbers on this dial are only approximate. Use a thermometer to check
the actual temperature of the tank, and make slight adjustments to the
heater accordingly. The sticky LCD thermometers for example are cheap,
reasonably accurate, and very easy to use.
Is the heater broken? Again, a thermometer will help you know if the
heater isn’t doing its job, which is why they’re a small but important
part of the aquarist’s toolkit. If the water is too cold, the heater
might be set too low, but it might also have failed entirely. As with
any appliance, check the fuse in the plug before throwing out a
seemingly ‘dead’ heater.
If the water is too hot, and turning the dial on the thermostat doesn’t
help, then the heater may be jammed ‘on’. This is rare, but does happen,
and can end up killing your fish if the water overheats too much.
Because of this, it’s always worth buying a heater from manufacturer you
trust rather than a no-name heater cheaply available online.
One last thing: condensation inside the heater is not normal, and a sign
that water is somehow seeping into the heater from the aquarium. Given
these are mains voltage appliances, dodgy heaters should be immediately
switched off at the mains, removed and replaced. That said, good quality
heaters that aren’t pushed too hard should last for more than ten years.
Problems with poisons and other chemicals
Fish can be exposed to toxins in two ways, either dissolved in the water
or in the air above the tank. In either case the aquarist can suddenly
find themselves with an entire tank of fish gasping at the surface,
rolling about unable to swim or balance properly, or simply dying within
a few minutes of exposure.
Has something poisoned the water? Obviously cleaning products such as
bleach and detergents must be kept out of the tank, anything used to
clean items meant to go in the tank will need to be completely rinsed
away. You also want to ensure nothing goes into the tank that isn’t 100%
aquarium safe. While it is possible to collect your own rocks and
bogwood, you do need to know what you’re doing, and for casual aquarists
it is simply safer to rely on material sold in pet shops and garden
centres for use in fish tanks and ponds.
Very occasionally things fall into tanks, or are deliberately put in
them by ‘pranksters’, that end up doing harm. Anything containing
copper, for example, certain coins and screws, will release copper into
the water that is highly toxic to most forms of aquatic life, including
fish and shrimps.
Has something poisoned the air? Airborne toxins are less of a risk most
of the time, but if you’re doing home improvements or a serious spring
clean, then this possibility will need to be considered. Some types of
paint and many cleaning products contain chemicals that will be stated
on the packaging as harmful to people and pets, and these are almost
certainly going to be dangerous to fish, too.
Ideally, you’d not use these sorts of products in the same room as the
fish, but if that isn’t an option, and moving the aquarium isn’t an
option either, then at least minimise the risk by opening some windows
and ventilating the room as well as possible. Switching off any aeration
devices in the aquarium will minimise contact between the air and water,
and turning off the tank lights will trick the fish into becoming less
inactive, reducing their need for oxygenated water a bit. It’s also a
good idea to throw a sheet or towel over the tank as well, to make sure
there’s no chance of droplets of paint or cleaning solvents getting into
the water by accident.
Problems with plants
It’s very common for aquarists to have trouble getting certain plants
established in their tanks. As with fish, plants have their own
requirements, and if you tank doesn’t have the right level of light, or
the wrong water chemistry, then those plants will never do well. But
what about the situation where a planted tank worked well for years
without problems, but suddenly things went downhill?
Have the plants been declining slowly for a while? If your plants were
happy until recently, but over the last few weeks or months have grown
more slowly or become etiolated, then the chances are there’s not enough
light in their world. Fluorescent tubes designed for plant growth having
a working life of between 6-12 months depending on the brand. LEDs last
many times longer, ten years or more, so while they’re more expensive up
front, over the long term they’re actually better value.
The other reason for a slow decline in the health of aquarium plants is
the absence of mineral nutrients. Plants suffering this way often turn
yellow as they try to move minerals from older leaves into newer ones,
essentially recycling iron or magnesium as best they can. Gravel doesn’t
contain any minerals that plants can use, so the use of fertiliser
tablets will be essential in tanks where gravel is used. But even the
high-end substrates designed expressly for planted aquaria will ‘run
out’ of minerals eventually. Replacing such substrates isn’t practical
without rebuilding the tank, but there’s nothing to stop you adding
fertiliser tablets to these substrates should you need to.
Was the decline sudden? Plants tend to react to changes in their
environment slowly when compared with animals, but there are some
situations where this isn’t the case. For example, Vallisneria and
Cryptocoryne species will ‘melt’ when severely stressed, though for
different reasons. Vallisneria dislike sudden drops in the pH, being
plants happiest in hard water conditions. Cryptocoryne are notoriously
sensitive to being uprooted or moved from one tank to another. It may
take some weeks for such plants to recover.
Another reason plants can suddenly look in terrible shape is physical
damage. If your plants are looking raggedy, it could mean that a fish
(or even a snail) has started to eat them. Quite a few commonly kept
fish are confirmed plant-eaters, including many of the larger barbs,
some cichlids including Severums and Uaru, and most of the medium to
large characins, including Distichodus, Anostomus, and of course the
Silver Dollars. While most Plecs ignore healthy plants, there are some
genera, including Panaque, which will eat aquarium plants. Review your
stocklist and act accordingly!
Hopefully this gives some insight into the major ways a good aquarium
can go bad. If all else fails though, do feel free to write to the Crew
here at WetWebMedia and we’ll see what we can do to help!