Hi, I am new to coldwater fish and recently purchased a 75 tall (48" x 18" x 21").

My mother has a small pond in which she keeps (too many) large goldfish. I offered to house as many as possible. The fish are probably 10-12" including their tails. I think they are almost all comets.

How many of these would it be possible to keep in my new tank? I know that they are fairly 'dirty' fish.

What is the most effective way at keeping the tank at an optimum temperature? I know there are chillers sold for this purpose? Any insight into this equipment is very welcome as I know nothing about chillers.

What is the optimum/acceptable temperature range for goldfish?