help dying fish

Question
QUESTION: Hi,
I had a 5 1/2 gal. tank with platy fry. Yesterday I found all of the fry hanging out by the filter. I checked the chemistry and everything was fine but the PH was low. I did a partial water change and then raised the PH. The fish seemed fine and were eating well after. This morning I found several of them dead. I looked at the fish and they have no signs of problems (tails, gills, scales etc.) and look fine until they swim frantically into the tank and die. Please help.
ANSWER: This is a hard question to answer because there is no proof of how they died, you said here water chemistry was good. The only thing i suggest is in a 5.5 gallon tank you need to be doing frequent water changes, every day, or every other day. What were you feeding them? Fry do best off newly hatched brine shrimp, they provide the beat amount of protein to offer to fry. I'm not sure what the survive ratio is but you can expect some of them to die, it's just natures way of cutting out the weak ones (darwins) survival of the fittest. All i can suggest is frequent water changes and brine shrimp. You also should be running a filter (sponge filter connected to a airline), you need some sort of biological filtration in there. That's all i can mention.

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QUESTION: Hi thanks. I guess I should have been more specific they are not exactly fry but juvenile fish (approximately 1 week to 1 month). The younger fish are fine and the older ones are dying. I have a bubble air stone and a biowheel filter. I ran out of live baby brine shrimp and substituted with a "artificial artemia." When I fed them two days ago I accidently gave them too much. Could that have done it and if so what could I do, if anything.
ANSWER: Ok i undersatnd now, interesting. I would suggest you retest your water and test for ammonia, nitrite and nitrate. How long has this tank been going for, maybe your tank is still cycling and your having ammonia spikes. That could be a reason, if so water changes daily would help. I dont think that it's caused cause you overfed them that one day.. very unlikely. I really dont have much to go on though. Do you have any other fish in the tank, if so, are they doing ok aswell, or is it just one species

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QUESTION: I tested again. The ammon, nitrate, nitrite are at 0. The PH is still a little low 6.5. I have Platys and guppies in the tank and so far only the Platys have died. The tank is approx. 3 weeks old. I really appreciate your help, thanks.
ANSWER: I dont know why your readings are all at 0, they should register as somthing espeically at 3 weeks. At least you should have nitrite readings.... I think your tank is going through ammonia spikes which is part of the intial cycle. I think that's exactly where your problem is.. your fish are stressing and dying off like that. It's going to be hard to save the fish unless you have beneficial bacteria from someone elses tank you can put in. Or another tank that's already fully cycled and setup.

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QUESTION: I just lost another. He ran full force into the side of the tank...sad. The gravel from this tank was taken from a smaller cycled under gravel filter tank. So I think the bacteria is present. I have another tank I can put them in but I am afraid that if there is a disease of some sort I could kill off my adult fish.  

Answer
Hmmm... that's werid, i'v never heard of fish just committing suiside. I think it has somthing to do with the water, espeically if the fish have no present symptoms... besides running into the glass.. sorry dude, not sure anymore. I'm still convinced it's somthing with the water. Hows your temperature?