My fish are dying

Question
Hi sir,

I am living in New Delhi, India.  I have recently (2 month ago) bought a 2 ft. acquarium.  I also bought 4 gold fish and 2 dollor fish.  In a few days it started dying.  I do change the water of my tank once in a month and during noon time.

I am regularly giving food them.  I am using anti clorine medicines, ith medicines etc when i cange the water.  But even then, they are dying.  But after some time, (15 days) my water gives bad smell.

I am currently using water filter and oxygen pump in the tank.  

Pls help.

Answer
Rohit,
The first problem I see is that you have the wrong fish together. Goldfish are cold water and silver dollar are tropical. That means the goldfish do not need a heater but the other does. Secondly, your tank is way too small to even house one goldfish. To keep 4 of them healthy and happy, they can be in a tank no smaller than a 60 gallon. Water changes need to be done weekly regardless if you have a filter or not. Waiting 1 month is making the fish swim in ammonia and nitrites which are the chemicals found in fish waste. Because the tank is so small 25% water changes will have to be done daily until they are in the right size tank. Also, you have to cycle the tank before you add any fish. The cycle process can, and often does, take 6 weeks.If you add fish to a tank that has not been cycled, they will die. So, here is what I suggest to help the problem:

Do not keep cold water fish with tropical fish.
Buy the right size tank for the fish you are going to keep
Research what your fish need, ie: water conditions, food, tank size, compatible tank mates
Do not put fish in the tank until it is cycled.
Do not add any medication to the tank unless you know exactly what is wrong.

Doing all of the will help you keep your fish happy and healthy