Why Are My Fish Dying?!

[allydawn17]

I have lost 7 fish in the past 3 days in my 55gallon tank. I recently added some fish because it has just finished cycling, (0 ammonia, 0 nitrites, ~20 nitrates). I originally had 2 cories and 1 angelfish, and I then added 6 more cories and a bn pleco, and had no problems. A few weeks later (last weekend) I added 10 diamond tetra and a pair of German Blue rams, and some new plants. I have since lost 4 cories, both of the rams, and a tetra. They have shown no signs of illness or anything at all, they are perfectly fine up until the moment I find them dead. Testing my tank now I see .25 amonia, .25 nitrites and around 20 nitrates. I blame the slight rise in ammonia and nitrites on the dead fish, and the amount of fish I added at once. I assume that whatever this is came home with the new fish and/or the plants, but considering there are no warning signs I don't know what to treat for or if I should quarantine anyone. Please help, I don't want to lose any more fish!

 

[fluttermoth]

I think can all you can do, in the absence of symptoms, is keep the ammonia and nitrite as low as possible with water changes and keep your fingers crossed.
 
Sorry for your losses, it's always upsetting

 

[BerryAttack]

you added way to many fish in.. which caused a spike and killed the others.. throwing you tank off.
stop buying fish.. keep the water clean as possible to remove ammonia etc.
 
when you get the under control wait a while before getting fish..... around 2 weeks after everything is perfect. then only get 6 fish at a time... then you have to wait another 2 weeks before adding in another group(s) of fish.. 

 

[Catfishlover]

Watch the nitrates; they can kill fish if they get to high

 

[sawickib]

Nitrates are the least of your problems, ive had very high nitrates above 160ppm and not had very large problems although it should never reach this much and im a shamed it happened but it happened, gave my BGK pretty bad fin rot but fixed it with water changes. nitrite and ammonia is much more lethal to fish, it is a larger chemical bond and doesnt pass through the gills very well, and no matter how well oxygenated your tank is they can sufficate or suffer from ammonia burns and other conditions.

 

[Meeresstille]

It's too late to do any quarantining now. Quarantine is a good practice to observe new fish for 6 weeks before adding them to the main tank, so any diseases they might have will not infect your other fish. A hospital tank is only useful if you know which fish is sick, so you can keep it away from the other fish and to treat that fish without having to treat healthy fish as well. 
 
The amount of ammonia does not sound lethal to me (what is your pH?), nitrite poisoning sounds plausible, but you said there were no symptoms the fish were having. With nitrite poisoning fish will stay close to the surface, gasping for air! Do you see any white spots on any of your fish? I'm thinking ich is a good possibility since new fish can bring an ich infection with them, and ich can settle in the lungs which can kill a fish quickly. 
 
Your Nitrate levels look high for a new tank, are you perhaps feeding too much? Do good gravel vacs to pick up uneaten food and fish waste, it will keep the water quality better as well as the nitrate at good levels!
 
Also, German Blue Rams are extremely sensitive and need pristine water conditions! I would wait until the tank is well established before adding this type of fish. Then you will have to keep very good tank hygiene to keep water parameters stable, these fish don't adjust well to fluctuations.