I added 4 female and 2 male silvertip tetra's to my 55 litre (12 gallon?) tank on the 27th July. The fish stayed together for a few hours but slowly went off to explore individually once they felt comfortable. Everything seemed great, they were swimming/feeding fine (flaked food) and seemed happy. Over the last few days though i have noticed that one fish in particular begun isolating itself from the group. It's always hiding at the very back of the tank near the plants. It's also losing a little colour mostly in the black stripe near it's tail. My partner and i are really worried that it's stressed due to constantly being chased by the other fish. When the other fish come near it, it's moving away to another hiding spot. Is it possible that it's being bullied? It very rarely swims around anymore, only when being chased away from it's hiding spot or when food is put into the tank. We're not sure what to do as this is our first aquarium. We bought the tank on the 18th July from Pets at home. We were told to set the tank up, leave it a week and then begin slowly adding fish. We were given a product called 'tapsafe' to ensure the tap water is safe, though weren't informed about testing for nitrate etc. We'll need to purchase a tester immediately.