If you want comets, you should go with a tank longer than 6 feet. Those guys really like to swim and fast. Standard glass tanks that come in this size are the 125 gal, 150 gal, and 180 gal. Of course, bigger is always better with
Goldfish.
In a 55 gallon (which is only 4 feet long), you can't do any more than four fancies. It becomes very hard to keep the nitrates at manageable levels beyond that. Swimming room also becomes a problem at those stocking levels as even the fancy varieties can grow 8 inches long (not including tail). They are big and clumsy and need room to maneuver.
In a 75 gal (which is the same length and width as the 55 gallon), you could do six fancies. But I would build the stocking levels up slowly and keep an eye on nitrates. They are toxic to fish in high levels. Plants are helpful at keeping these down, but not all
Goldfish cooperate with you on this.
Comets grow larger than fancies are are more active than them. One or two could be kept in a 4 foot tank for a few months, but you'll soon find them bouncing off the walls and bumping into the fancies.
Also, I'm not too sure the crayfish is a good idea with the cumbersome fancies. I don't think they could move fast enough to get out of his way.
