Why are my fish dying?

It sounds like you may have high levels of nitrite in your tank- which would suggest that the tank isn't really fully cycled yet.

When you cycle an aquarium, you will first see a spike in ammonia, then the ammonia will drop to zero and you will get a nitrite spike (this is probably where you are) and then the nitrite will drop to zero and you will start getting positive readings for nitrate. That is when it is safe to add fish and just do weekly maintenance.

As you do have fish in this tank, I would suggest that if you don't already have one- buy an API liquid test kit and test your water. If you get positive readings for ammonia or nitrite, your tank isn't really cycled. Any time you get .25ppm for ammonia or nitrite, do a 25-50% water change. For .5ppm do a 50% water change and anything above 1.0ppm do 2 50% water changes daily.

It is really important that you test your water every day when you are cycling a tank with fish in it!

I would suggest taking a sample of water out of your tank into the pet store and ask them to test it for you. Could be something in your water, especially if your using tap water. If your getting all these fish from the same pet store you might ask if they've had customers complaining lately of this (they probably wouldn't tell you if they had but never hurts to ask.)

The bent spine is a classic symptom of nitrate poisoning. The loss of colour indicates parasites. Considering the nitrate would be causing extreme stress, lowering the fish's immune system response, it is probable that there was a secondary parasitic infection by the bad guys that are in every tank. The most common is flukes.

Water tests will tell you more.

Running a tank for a month may not help the cycle at all if there is no source of ammonia to feed the bacteria you need. Then you add a full population of fish, and the cycle process starts then, with way too many fish for an uncycled tank to support. The numbers from any useful guide are for a stable and established tank, not a new one.

So what you probably have now is very poor water quality, that stresses the fish and leaves them vulnerable to all sorts of random infections and random deadness.

What to do? Increase the water changes, up to 50%, and up to daily. This will temporarily improve the water quality, and may be enough to allow the rest of the fish to survive while the cycle process completes. There are various antibiotic medications that will help fish fight off those infections, and a water test kit will let you monitor what is going on. Then you will see when the water quality improves, the cycle gets established properly, you can ease off the water changes, and replace the fsih you lost.

This page explains the nitrogen cycle properly, and will help you undersand what has gone wrong, and how to fix it.

http://www.fishlore.com/NitrogenCycle.htm

This is one of the most important things to know about aquarium keeping, but unfortunately most pet stores are not aware of it, or at least can't be bothered explaining to to beginners.

Ian

id say, theres some kind of problem in the fish. Separate them all in separate bowls and such, with new water, and see if it still happens, or just completely redo the water altogether. Good luck

I suspect high nitrites but it could be any number of other problems that are hard to diagnose without reviewing all your maintenance habits, water tests, chemicals you use etc. etc..