temperate aquarium

I bought a biorb 15 litre when I knew nothing. I saw the WCM on the front and thought great!! Until I research :( anyway, I decide I would buy a fighter. My tank is cycled and I went to fit my heater today and the tank is too small (I here you say surprise I KNOW!!!!!! :( ) Anyways, I know...

Hi. I am about to start restocking my temperate fish tank. It was previously filled with variatus platys, but most of them have passed on after about 2 years with me (their lifespan is 2-3 years isn't it?). Anyway, it is a 90l (20 UK gallon) tank with a filter, lights, a few plants and other...