2. 5 tank to small for 2 shubunkin goldfish?

I've asked many questions on here about my new fish and I've have some people say this sized tank is fine and other say I'm killing them slowly basically. I've tried to take them back and I can't they have been living for 2 weeks so far I have a filter and I am going to clean it every fortnight like the petshop told me. The guy who sold my mum the fish said this tank was fine twice but I'm worried about them and I can't take them back now I didn't even know I was gonna get fish I was staying away and when I come home my mum got them for me as a welcome back surprise. But when those fish do die and if we decide to get new fish any reccomendations because we just can't afford a bigger tank at the moment