How often should I clean my fish tank?

I recently got some fish: black moor goldfish, 3 comet goldfish and algae eater and one more.. and i'm not sure but i think the other one is a fantail. Anyways I have a pump with it, but how often should i clean their tank? Should I get a filter? I'm not sure how many gallons it is but it took me 5 12 litre buckets to fill it. Oh and any way to measure the gallons of my tank? hahah thanks x