Oh, whew, I thought a Fluval 6 was a 6 gallon tank! But 90 litres (24 gallons) is slightly more reasonable
As for being harsh, I'm not trying to be, it's just facts. Fish can be deformed and lead pain-filled, shortened lives by being kept in too-small tanks. Not to mention being poisoned or burned by their own wastes.
To bring it back to hamsters, if someone came on here and said they loved their hamster but only had room for a shoebox that couldn't fit a proper wheel, it's likely the members here would recommend rehoming it for its own well-being.