hi its usually recommended not putting a tank near a radiator - but
Firstly you can turn down the radiator so it doesn't get as hot.
Secondly you could insulate the tank - the insulation would stop the tank warming up to quickly as it would protect the tank from the heat and also keep the tank temperature more stable. Also if you are contemplating tropicals they are accustomed to some temperature variations but keep it gradual - hence lowering the radiator. Hope this helps