real emergency here...all messed up

Question
ok..this morning before i went to school, i checked my fishes, ALL seems alright but when i came back home in the afternoon, i was shocked that 1 silver dollar and 1 platy was dead...immediately, i did a 40% water change and realised that some of my remainning platys had finrot while my other silver dollar had velvet...i took the silver dollar out and put in a hospital tank with some velvet med while i treated the main tank with fin rot med...even though id id a 40% water change, de ammonia lvl was still 0.25ppm...and the last time i did a water change was 6 days ago and at that time, all water stats: ammonia, nitrite and nitrate were ALL good...with the exception of nitrate, all were 0...now i do not know what had happen...but all my fishes are stll eating well.. i also added some salt in the tank...what can i do?

ammonia:0.25ppm
nitrite:0.1ppm
nitrate:15ppm

tank size:20 gall planted
remaining tank inhibitants: 4 platys, 1 clown plec, 5 corys, 1 clown loach and 1 silver dollar...

tank equipments: 1 HOB filter with bio wheel, 1 internal filter and 1 air pump with air stone

what should i do?..thanks

Answer
Hello Jun:  I have a couple of questions for you... how often and what do you feed the fish?  How long have you had them?  

If the tank is new... less then a month old then I would add more beneficial bacteria.

If the tank is older more then three months old... then I would clean the gravel and make sure that fish food and fish waste have not collected under the ornaments.  You chemical readings indicate that the tank has an active nitrogen cycle in progress and that is ok.  The trick in this case is to break that cycle by removing the source of the ammonia.  Ammonia is the byproduct of decomposition and that is why the gravel vacuum is necessary.  At your current water chemistry levels the fish are not at all in danger.  The death of the silver dollar and platy may be more stress related...especially if they are new to your tank.  I would also boost their diet and offer them a variety of foods such as flake food, brine shrimp and tubiflex or blood worms.  A well balanced diet will boost their immune system.  Healthy fish rarely get sick and die.  Keep me posted... dave