Don't overfeed the fish.
Unlike mammals and birds that use most of the food they eat to keep warm, most fish take their body temperature from the surrounding water. This means anything they eat is used for movement and growth. This allows fish to get by with less food than most people think.
Over feeding regularly causes water quality issues (ammonia and nitrite problems), and is a major cause of problems in newly set up aquariums.
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Read up on the filter cycle.
Don't add too many fish too quickly. Everyone wants fish in their tank but unless the tank has an established filter, you don't want to add too many fish. If you cycle the aquarium before adding fish, then you can add them all at once after the filter has developed the good bacteria. But it takes about 4-6 weeks to cycle a tank.
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Live Plants.
Live aquarium plants are always good but need light and a bit of aquarium plant fertiliser to get them going.
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Diseased Fish at the Shop.
Look at the fish in the shop tanks and see if any look unwell. Look for fish that have cream, white or grey patches, or white spots, damaged fins, cloudy eyes.
Look for fish breathing heavily or gasping at the surface, that's a bad sign.
Look for fish that are sitting alone in a corner when they should be in a group. Sick fish often swim off on their own and wait to die in a corner. This is more for tetras, rasboras and barbs.
These are all health issues and you should avoid buying any fish from that tank.
There are some health issues you won't see in fish but they might have. intestinal worms and gill flukes are 2 common parasites found in the fish's digestive tract, and on fish's gill filaments. You can't normally see these but certain fish (guppies, platies, swordtails and mollies) just about always have them and they should be treated when you get them.
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TB and the Iridovirus.
Don't buy dwarf gouramis (Colisa lalius) or any of its colour forms because they regularly carry the gourami Iridovirus and or Fish Tuberculosis (TB). Neither of which can be cured and the disease remains in the tank until you strip the tank down and disinfect it.
Rainbowfish from Australia and New Guinea can also have issues with Fish TB so try to buy eggs or get fish from people who have healthy stock. Unfortunately it's hard to tell if rainbowfish have TB, but they sometimes get ulcers on their body, and if any rainbowfish suddenly bloats up over night, stops eating, does a stringy white poop, and gasps at the surface, there is a chance they have it.
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If in Doubt, Water Change it Out.
If you have any concerns about the fish, eg: you think they look off, test the water quality for ammonia, nitrite and nitrate, and then do a big (75%) water change every day for a week.
Make sure the new water is free of chlorine/ chloramine before it's added to the tank.
The following link has information about what to do if your fish are unwell. It's worth a read when you have some spare time.
If your fish ever looks sick or unwell, then the following steps might help. Test the Water and Clean the Tank. Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but...
www.fishforums.net
