Hardy fairground fish outliving newly bought fish

[WelshMoth]

DD has a tank with a filter and her pride and joy fish friend that she won last year in a fairground.

We've bough it a little companion from my local aquarium and very expensive over-priced brand garden centre TWICE and each fish has only lasted about 6 weeks.

We've carefully added the drops to the tank and she feeds them a tiny pinch carefully each morning and night.

What are we doing wrong? She's distraught every time one dies but now wants a third so that Lucky the Fair ground fish isn't lonely.

[kalopali]

How big is the tank? Have you tested for ammonia, nitrates and nitrites? How are you cleaning the filter media? Are you treating the fresh tap water when doing water changes?

[Lifeover]

Is the original fish a goldfish? If so you need a large tank to add another. How long are the fish lasting? Is the original fish killing them?