A lot of the problems that get asked may be new to you, but they aren't new to us! Or put another way, you'll get a quicker response to your fishkeeping emergency by using the search tool to review existing articles and FAQs.
But help speed things up, here's a review of links and explanations related to the most common problems.
1. Tank quite new, but lots of fish are dying!
Almost certainly down to poor water quality. The biological filter takes about four to six weeks to mature. While you can speed this up in various ways, it is still easy to overstock/overfeed and end up with poor water quality.
2. Tank is mature, but lots of fish suddenly dying
There are three likely causes: poor water quality, rapid changes in water chemistry, or the accidental poisoning of the water. It is relatively rare for a killer disease to suddenly spring out of nowhere, though if you recently introduced some new fish, they can certainly have brought diseases with them (which is why you should quarantine new livestock).
Use an ammonia or nitrite test kit to check water quality. Are you are feeding the fish? Is filtration is adequate? Is the tank overstocked? Are you doing enough water changes? Most community fish are adaptable in terms of water chemistry, but use a pH test kit to ensure water chemistry stays stable. If necessary, incorporate chemical buffers to maintain pH. Possible poisons include pesticides, paint fumes, and other such chemicals.
Small tanks (37 litres/10 gallons or less) are difficult to maintain and easily overstocked. Water quality and water chemistry will fluctuate quickly in these systems, stressing your livestock. If you have a small tank, upgrading to one above 70 litres/20 gallons in size will dramatically improve your hobby (as well as the lives of your fish).
3. Some of my fish are sick
The vast majority of times fish get sick, the disease involved is whitespot, velvet, finrot, fungus, or the shimmies. Use this article to establish which disease is causing your problems, and then select the appropriate treatment.
Note that most of the WWM crew aren't wild about tea tree oil products such as Melafix and Pimafix. At best these are unreliable cures, at worst completely useless. There are ample, medically-tested products out there: use those instead.
4. Livebearers (Mollies, Guppies, etc.) sick
All Central American livebearers need hard, basic water. If kept in soft, acidic conditions they are prone to diseases such as finrot. Mollies are additionally very sensitive to nitrate, particularly if kept in freshwater rather than brackish or marine conditions. Mollies are therefore most easily maintained (and some would say only ever reliably maintained) in brackish water with at least 3-6 grammes of marine salt mix per litre (SG 1.003-1.005). Because Mollies, and to some degree livebearers generally, have very particular water chemistry requirements, they are best maintained in their own aquarium rather than a standard community system.
5. Neons dying one at a time; everything else is the tank is fine
Neon tetras are prone to a disease known as Pleistophora hyphessobryconis or Neon Tetra Disease (NTD). The symptoms start with shyness and loss of appetite, the infected neon usually leaving the school and hiding in a dark corner. Gradually the colours fade to off-white, the fish becomes lethargic, either bloated or emaciated, and then dies. There's no cure. The best you can do is isolate infected fish to prevent infection of healthy fish. Observe the fish carefully, and if the symptoms match NTD, painlessly destroy the fish. Pleistophora can infect species other than neons, though infrequently.
Neons can of course become sick because of other things such as finrot and fungus, so simply because you have a sick neon, don't automatically assume it's Neon Tetra Disease.
6. Dwarf Gouramis are becoming emaciated, losing their colour
Dwarf gouramis are prone to a virus known as the Dwarf Gourami Iridovirus. One study by vets found that 22% of the dwarf gouramis exported from Singapore were infected with the virus. Infected fish gradually weaken and lose their colour, eventually developing sores and lesions on the body. The faeces become stringy and pale. The virus appears to be 100% fatal and completely incurable; it is also highly infectious. Sick fish should be isolated at once, and if Dwarf Gourami Disease is confirmed by careful observation of the symptoms, the fish should be painlessly destroyed.
Dwarf Gouramis may become sick for a variety of other reasons, and diseases like finrot and fungus can create superficially similar symptoms. That said, the quality of dwarf gouramis on the market is very variable, so take great care when shopping for these fish. Ideally avoid them altogether and opt for hardier, easier to maintain species such as Colisa fasciata and Colisa labiosus.
7. Fin-nipping, or some of your fish have ragged fins
Ragged fins can be a symptom of finrot, but if your aquarium enjoys good water quality and the fish seem to have had bites taken out of their fin membranes, fin-nipping may be a problem. Multiple so-called community tank species are known to be fin-nippers and should never be kept with slow-moving or long-finned species (such as angelfish and guppies). Among the most common fin-nipping fish are various (though not all) Hyphessobrycon spp. (most notoriously serpae tetras and jewel tetras); Gymnocorymbus ternetzi (also known as petticoat tetras, black tetras, and black widows); and Puntius tetrazona (also known as tiger barbs).
8. Snails everywhere!
Snails turn uneaten food and other wastes into baby snails. If you have problems with snails, then the first step in fixing things is to establish what they're eating. Avoid using snail-killing potions: having lots of dead snails rotting in your aquarium will reduce water quality dramatically. There aren't any completely community-tank safe fish that eat snails, though loaches and thorny catfish (family Doradidae) will eat snails if they are sufficiently hungry. But do be aware that loaches are gregarious and often boisterous fish, and thorny catfish will also eat very small fish including neons.
1. Fins becoming ragged
Ragged fins are most frequently caused by finrot and/or fungus. These are both diseases directly triggered by poor water quality. If your betta is kept in a community setting, fin-nipping can be a problem. Bettas are best kept alone.
2. Not eating
Bettas will lose their appetite when kept improperly, i.e., in unheated and/or unfiltered water. Otherwise they are easy to feed on a wide variety of foods. They do appreciate variety though, so don't stick to just one type of food, such as pellets. Use live foods and frozen foods liberally to avoid constipation and keep your betta healthy.
3. Unheated and/or unfiltered aquarium
Bettas are tropical fish and can't be kept at room temperature (unless of course you maintain your home at a constant 25 C/77 F. Keep them too cold and they'll get sick.
Breeders often keep them in bowls in heated 'fish rooms'. To keep the water quality good, they will be performing daily water changes, often changing as much as 50-90% of the water each time. Unless you will do the same thing, don't even think about not using a filter. Inexpensive air-powered sponge filters will make life much easier for you and your betta.
4. Kept in a 'betta bowl' or similar tiny aquarium
Bettas can survive in very small tanks for long periods, but realistically they do best in at least somewhat generous quarters. Aim for 18 litres/5 gallons, and you'll find your betta a much happier fish (as well as easier to keep).
1. New set up, goldfish already sick!
Almost certainly down to poor water quality. A biological filter takes about four to six weeks to mature. Because goldfish are greedy fish, it is very easy to overfeed and end up with poor water quality. You'd also be surprised how much space and filtration goldfish need: reckon on 115 litres/30 gallons for a basic goldfish system. Keep them in anything less and you're asking for trouble.
2. Floating, bloated goldfish
Goldfish are herbivores; they will not do well on an all-flake or all-pellet diet.
1. Laboured breathing, wheezing, mucous from the nose/mouth
Respiratory tract infection; very common when turtles kept improperly, for example too cold or without a UV-B lamp for basking. Needs prompt veterinarian attention as well as a review of diet and housing.
2. Running or swollen eyes
Bacterial infections of the eye; very common when turtles kept improperly, in particular when the diet is lacking in Vitamin A. Needs prompt veterinarian attention as well as a review of diet and environmental conditions.
3. Shells smelly and/or rotting
Shell rot is a common problem when turtles are kept in dirty water; a source of UV-B light is also essential for proper growth of the shell plates. Treat/clean the shell, and then fix the environmental conditions.
4. Starting out, need help
See here. Bear in mind most turtles are large and demanding pets, as well as a potential source of Salmonella infections; turtles are not even close to being good pets for children!
5. Green foods for turtles
Many freshwater turtles are omnivores and eat a substantial amount of plant material in the wild; plants account for 50% of the diet of juvenile red-ear sliders in the wild, and more than 75% of the diet of the adults. Green foods are cheap and easy to use, so there's no reason not to use them.