Pls find my below comments.We'll need some information to be able to help. How long have you had the tank? How long have you had the fish? What size is the tank and what kind of filter are you using? What are your water parameters: ammonia measurement, nitrite measurement, and nitrate measurement? Have you read about the nitrogen cycle? What symptoms are your fish showing?
I am a newbie & I have bought 20 gallen tank 2 days back and bought this guppy fish also 2 days back. I have put the water from a water can supplied by the local water supplier. I have just filled 1/5 of the tank volume with water. I havent started the air pump that comes in the build of the aquarium and I am using an external boyu water pump with airstone. There are only few small fishes like 6 in number and they are getting the rotten tail. Today morning one redtail guppy fish was having tail rot. The tail turned from red to black and something it was looking like torn and now at 10pm the fish is dead. Some other guppies in tank are about to get the tail rot. I thinkchanging water will not work. Idk what medic to use and local aquarium shopkeepers are also not responding properly.
The article looks very brief and useful. I will go through it.Okay, here is a good article about starting a freshwater aquarium. http://www.aquariumadvice.com/guide-to-starting-a-freshwater-aquarium/ At this point I think that lots of clean water is the best medicine. You should fill the tank all the way and should do lots of partial water changes with good clean water. At this point 50% water change every 2- 3 days would be a good amount. If you are not using a water conditioner you should do so; these will remove chlorine and chloromine that may be in the water.