What kind of fish did you introduce, and what kinds of fish are in the tank already?
Did you cycle it before you set it up for at least five days?
How long has it been running?
We need this information in order to pinpoint the problem. It is fairly obvious that your tank has some kind of parasite/bacteria that is killing your fish, and you should go to a local pet store and see if they have any anti-bacterial products to use as daily drops for your tank. It could be that the newly introduced fish carried some kind of parasite.
Did you add all thirty fish at once? This could be part of the problem, as only a maximum of three fish should be introduced to any tank environment at once, then a five day wait should happen to see how they are adjusting, then add three more, etc. Adding too many fish at once could throw off the balances of your water, making the parasites.
Did you cycle it before you set it up for at least five days?
How long has it been running?
We need this information in order to pinpoint the problem. It is fairly obvious that your tank has some kind of parasite/bacteria that is killing your fish, and you should go to a local pet store and see if they have any anti-bacterial products to use as daily drops for your tank. It could be that the newly introduced fish carried some kind of parasite.
Did you add all thirty fish at once? This could be part of the problem, as only a maximum of three fish should be introduced to any tank environment at once, then a five day wait should happen to see how they are adjusting, then add three more, etc. Adding too many fish at once could throw off the balances of your water, making the parasites.