Please visit our Sponsors

Related FAQs: Going from Freshwater to a Marine Aquarium, FAQs 2Freshwater to Marine 3,

Related Articles: Marine Aquarium Set-up, Components

Converting From Freshwater to Marine Aquarium Keeping

by Anthony Calfo

Death traps

After some months or even years as a freshwater aquarist, it is inevitable for many folks to ponder what it would take to convert their freshwater aquarium to a marine system. Visiting the local pet store, reading hobby books and browsing the Internet, we see images of extraordinary and colorful saltwater species that tempt us to consider bringing them into our aquariums. To the uninitiated, the transition can seem a bit daunting when looking at the prices of the more exotic species and unique hardware. Truth be told, the conversion of a complete and successful freshwater (FW) aquarium to a saltwater (SW) ready system can be remarkably inexpensive and easy.

Any FW aquarium can be converted in an afternoon with most efforts costing less than $200, and many under $100. This claim may come as a shock to anyone that has priced some SW equipment and noticed single pieces of commanding prices in excess of $200. To clarify the statement, there are many types and themes of SW aquaria; some are as simple to conduct as your FW aquarium, while others require additional hardware and expense. A new marine aquarist can avoid, and will likely want to, the latter scenario of complex and dear systems. Mind you that in taking the conservative route you are hardly restricted from enjoying numerous colorful and amazing species of fish and invertebrates! You will simply need to resist at first the pursuit of delicate species and most photosynthetic invertebrates (corals, anemones, and the like). An exciting array of hardy and fascinating fishes, shrimps, starfish, crabs and more awaits you instead.

The key to a smooth transition is having a complete and successful freshwater aquarium to start with. Modestly outfitted systems may require additional expense. Aquarists that have had experience with FW species like cichlids (Oscars, Africans, Discus, etc.) and goldfish may find the husbandry needed for a SW aquarium unremarkable. The aforementioned freshwater fishes are notorious for their pressing demands on water quality as messy feeders or simply large fish in a heavy bio-load. A regular and consistent address of aquarium husbandry is necessary to maintain good water quality with both groups in kind. Small weekly water changes are better than large monthly ones. Monthly water changes are still quite acceptable with due diligence on other issues like careful feeding, understocking, etc. Using a basic water quality test kit is also advised for long-term success in any aquarium.

The three primary changes (additions) you will need to make to begin keeping marine species are: synthetic sea salt, a hydrometer to measure salt levels, and a new calcium-based (calcareous) substrate. The use of sea salt and a hydrometer is very easy and straightforward. There is no magic or mystery about making and maintaining safe salinities for marine organisms, but there are some very basic rules to follow:

Never mix sea salt in the aquarium with livestock present, as it is briefly caustic until it dissolves.

Always mix, aerate, and heat newly made seawater in a separate vessel for 12 or more hours in advance of use

Test, know and adjust your new saltwater to match the temperature, salinity and pH of the aquarium in which it will be used

Salt manufacturers give us a rough guide of measure (usually ? cup of sea salt mix per gallon of freshwater) to make seawater. Because of slight variations with mixes and water temperatures, however, only trust an accurate reading from a hydrometer to make saltwater. Hydrometers can be found affordably in glass and plastic varieties. A glass hydrometer is a floating tube that does not look or read very different from a thermometer. Inside the tube there is a calibrated scale, and the level at which the instrument floats (the waterline) is the correct salinity. More accurately, it is known as the specific gravity (SG) of the water. The "weight" or gravity of water increases as you add and dissolve salt, minerals, and etcetera into it. Thus, when sea salt is added to the water being tested, the density of the water increases and the hydrometer is displaced (physically pushed higher out of the water, which translates into a higher reading). Any reservations about how this simple instrument works are quickly put to rest when you use it at home on a sample glass or bowl of water. Add salt or extra freshwater and you will see the readings change on the unit. Glass hydrometers are fragile, but often more accurate to begin with and always more accurate over time than plastic hobby models. Plastic hydrometers are much more convenient and work with a simple levered arm that points to the correct SG with a scoop of seawater. Readings on plastic hydrometers can easily be corrupted though with air bubbles or a drop or sharp jarring of the instrument. All hydrometers can be affected over time if saltwater or hard tap water is allowed to repeatedly dry upon them, causing mineral deposits. Care for your hydrometer requires a simply rinse with distilled water after each use. For the value of the investment you will have in your marine aquarium, it's a good idea to have one of each kind of hydrometer for inexpensive insurance and verification of this critical parameter (both can often be obtained for $20 or less total).

Before you can measure your new saltwater with a hydrometer, the synthetic sea salt must be fully dissolved. Have a dedicated bucket set aside to mix and store new SW. Any food safe bucket will be fine. Most new plastic garbage cans are safe too. After adding sea salt, aerate and mix for several hours before testing and adjusting. An air pump with a coarse-bubble airstone, or a small powerhead with a venturi will do the job nicely. It is recommended that you run the mix overnight (12 or more hours) to be safe. Before using new seawater, your goal is to match your aquarium's temperature, SG and pH, primarily. Target a pH of 8.2-8.6, a temperature of around 78 F and a SG of 1.020- 1.026. It is a helpful trick in aquaria with fish only to lean towards the lower end of the salinity range while staying towards the higher end when keeping invertebrates. Natural seawater is around 1.025 while pure water is the standard for comparison at 1.000.

When adjusting temperature in the new mix or in the aquarium, never add hot water from the tap! Hot water is low in dissolved O2 and in rare cases can drive the oxygen out of your vessel to dangerously low levels. New seawater is to be heated slowly instead with a thermostatic heater. After 12 or more hours of heat and aeration, the water should be clear, the temperature adjusted, and the pH can be boosted if necessary with a small amount of baking soda or commercial sea buffer (coordinated with a test kit). Don't worry about the chalky carbonate residue that will coat the inside of your mix bucket… simply rinse it out between uses. At this point, your new mix is ready to use!

The third and last major change in the conversion of a FW aquarium is use of a calcium-based substrate like aragonite sand or crushed coral. It has long been said that one must have calcareous media to succeed with marine aquaria. Truthfully, there are many inert FW substrates that could be used safely, but enough others (painted, colored, unknown composition, etc) are unsafe or risky over time. More to the point, calcareous sand and gravel is especially helpful to the beginner for supporting pH and alkalinity in the system. This can prove to be crucial for the novice getting better acquainted with the feeding needs of new animals, and the need to maintain a high and stable pH in seawater. Calcareous media acts like a buffer in most systems. The issue overall is quite important and new to some freshwater aquarists. Unlike many FW fishes, marine species require a high and consistent pH. They hail from the enormous ocean environment, which is essentially stable and alkaline where tropicals are collected. Yet the natural tendency in any aquarium is for the pH to decline. These are some of the reasons why misinformed aquarists have believed that SW species are difficult to keep. With regular water changes, proper stocking and feeding, and occasional water tests, you will find that most marine species are no more demanding than freshwater species to keep. However, for busy or neglectful aquarists that cannot manage even monthly water changes, rarely test water quality and perhaps overfeed or overstock, FW fishes are more forgiving. This is largely based upon their natural tolerance for acidic, richly organic or aged waters like the many Amazon River fishes we commonly see in aquaria. With proper aquarium husbandry for all though, you will find that saltwater can be just as easy as freshwater!

If you are currently using an undergravel (UG) filter, you will want to seek crushed coral or aragonite of a comparable grain size to replace the substrate (aragonite is preferable). As with freshwater, a 3-5" bed of media over the UG plates is recommended. UG filters are not the ideal biological filter for modern marine aquaria, but they can serve a beginner very well when maintained properly. It will allow you to keep many hardy fishes and motile invertebrates like serpent starfish, shrimp or crabs without delving into too great of an expense for the conversion. If you have alternate filtration for your freshwater aquarium like a large canister filter, fluidized bed or trickle filter, you will be even better situated for the transition to keeping marine fishes. In such cases, a thin covering of calcareous sand or gravel (less than 1") instead will be most convenient. Be sure to clean, siphon or stir any substrate regularly as you would in freshwater. There are some fish that may be compatible with your featured specimens that will stir the sand for you as well.

After sand or gravel, the next consideration might be the "best" decorations and habitat for SW aquaria. Much has been written elsewhere about live rock, and new aquarists will be sure to read more of it in the future. In short, live rock is calcareous in nature (overgrown coral and calcified deposits from other organisms) and is infused with living organisms on and in it. Virtually all life forms carried with it are beneficial, and it is a source of tremendous fascination and utility for the marine aquarium. Live rock is an incomparable living filter, provides natural food for display animals, grows entertaining and fascinating plants and creatures, and is the quintessential natural habitat for most reef tropicals to be found. You are encouraged to use mostly live rock for natural d?or in the modern marine aquarium.

After the primary elements of a conversion have been addressed, the same fundamental principals of good freshwater aquarium husbandry apply to saltwater systems. Water changes should be conducted regularly. Chemical filtration like activated carbon is strongly recommended. Feeding regimes should address your animals specific needs and likely require small frequent portions of nutritively dense foods; avoid feeding hollow foods like adult brine shrimp as a large staple of the diet. You can put away any concerns you might have about the need for expensive light bulbs and lighting systems. Regular full spectrum lighting (6500-10K temps) in your standard light fixture will keep many or most of the desirable life forms on live rock, and illuminates your display fishes just fine. In time, you can upgrade to "reef lighting" when your knowledge and experience match your desire to keep some of the more demanding "flower animals" (corals and anemones). Do consider investing in a good protein skimmer though as soon as it is affordable. Be mindful than many of the entry-level skimmers are rather difficult to use if effective at all and you may be better served to focus on good water quality in wait for a better but more expensive model. Protein skimmers are very effective tools for improving water quality and reducing nuisance algae growths. All of these topics and so much more are covered in a competent aquarium handbook. Please buy or borrow and read a good reference on marine aquarium keeping to reinforce these concepts and enlighten you with many other ideas on how to finesse a beautiful saltwater aquarium in your home. See the checklist below for book recommendations and reminders. And best of luck with your salty endeavors!



Quality synthetic sea salt (remember: "Good things are seldom cheap, and cheap things are seldom good!")

Hydrometer to measure salt levels

Calcareous media: coral sand, aragonite, crushed coral, etc

Test kits: Ammonia, Nitrite, Nitrate, pH

A good book for reference like Robert Fenner's "Conscientious Marine Aquarist", Mike Paletta's "New Marine Aquarium" and the "Tetra Encyclopedia of the Marine Aquarium" by Dick Mills

Clean plastic bucket or can, sea buffer/baking soda, air pump, and an extra heater for making new saltwater


Protein skimmer (highly recommended but not necessary with light bio-loads and frequent water changes)

Second heater in display for redundancy and safety

Test kits: Calcium, Alkalinity, Phosphate

Additional recommendations:

Live rock

Battery operated air-pump for power/pump failures

Refugium and sump vessels for future system scenarios

Newbie article... whaddya think? Friends... I felt frisky and noticed a page/topic that need an article on WWM: Converting From Freshwater to Marine Aquarium Keeping http://www.wetwebmedia.com/fw2mar.htm I penned the attached with the intention of it being very beginner-oriented (for someone that knows next to nothing about marines). Any recommendations for improving it: content or language? Tried to keep it simple and short. Thanks kindly, Anthony

RE: newbie article... whaddya think? Hey Anthony,  Just got around to reading the FW -> SW conversion page and it looks great! I was wondering about a statement at the end of the required list. "A clean plastic bucket or can" Maybe it's just my Midwestern upbringing, but to me a can is metal. Can of beans, can of coffee, etc. I understand that you are writing 'plastic bucket or plastic can' but…..Maybe container? Or just plain 'plastic bucket'? My 2 cents Don

Small Marine Aquariums
Book 1: Invertebrates, Algae
New Print and eBook on Amazon:
by Robert (Bob) Fenner
Small Marine Aquariums
ook 2: Fishes
New Print and eBook on Amazon: by Robert (Bob) Fenner
Small Marine Aquariums Book 3: Systems
New Print and eBook on Amazon:
by Robert (Bob) Fenner
Become a Sponsor Features:
Daily FAQs FW Daily FAQs SW Pix of the Day FW Pix of the Day New On WWM
Helpful Links Hobbyist Forum Calendars Admin Index Cover Images
Featured Sponsors: