The tank is about 17-18 US gallons I think (70 litres). It's been set up just over a week - started with 5 lemon tetras and added 3 more today as the fish shop said it would be okay. There's no other fish in there Ammonia was 0.1 and nitrites 0.1 on Friday but haven't tested for Nitrates yet. We did about a 20% water change this morning. We feed the fish a very small pinch of flakes daily.
Noticed that the fish has a bit of a pinkish tinge on it's belly too.
70 litres = 18.49 US gallons
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The tank is cycling currently, see this article for more info on water chemistry in your tank and cycling explained;
http/www.fishforums.net/index.php?showtopic=10099
You should do a small water change (20% is usually good) with dechlorinator/water conditioner every time you see ammonia or nitrites in the tank as both these stats are toxic to fish and can stress fish out, which can lead to desease in fish or various stress related disorders. You should not buy any more fish until your water quality has stabilised, as adding more right now will just make the water quality less stable.
The swimbladder in the fish could be physically damaged (which would make it basically incurable), but it could also be an internal bacterial related problem or a diet related issue.
My advice would be to treat the tank with an anti internal bacterial med (like Anti Internal Bacteria by Interpet) and cut down on feeding the fish flakes or other dried foods and instead feed them a frozen based diet of daphinia (you should be able to get frozen cubes of daphinia/water flea's from your local fish store/lfs). Daphinia will be readily accepted by your tetras and is a healthy food that is good for fishes digestion when not fed in the freezedried form, but rather fresh/frozen.
Feeding the fish cooked, de-shelled and chopped up pea's is also good for their digestion, although this may not be as readily accepted by the lemon tetras as the daphinia, it is important to get some veg in their diet every now and then to help prevent diet releated problems like dropsy, constipation and swimbladder disorders. Always avoid overfeeding your fish, they should consume all food within a couple of minutes (there should be no uneaten food left on the substrate, particularly after 5mins).
So yeah, good diet and anti internal bacterial meds and regular water changes to try and help make the fish more comfortable during the cycling process should cure your fish. Swimbladder disorders are difficult to treat, but they are not untreatable- at the least, it should prevent your other fish sharing the same fate as your ill tetra
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