psmcgarry@mac.com :unsure:
My fancy koi goldfish"s tail looks like it went through a papershredder. Possibly my 10 inch long calico goldfish attacked him but I have never seen any aggressive behavior before or since Monday May 10th.
Do the tails grow back? Should I separate them? I currently have them sharing a 20 Gal tank with a good filter and an airline.

The tail may have been shredded by the other goldfish or the filter intake. It's possible that your larger goldfish is male and was chasing the fantail thinking her to be female (may or may not be). Male goldfish will beat up females during spawning and rip at the fins trying to eat the eggs. My poor pond goldfish are all cut up and fin-torn from being slammed into rocks, pots, plants, each other, etc.

The tail will grow back if the veins (the structure of the tail) is still intact. In that case, it could look almost normal when healed. If the structure is gone, they will never grow back normally but some regrowth can occur. Assuming fungus and bacteria don't set it, a goldfish can live missing some fins but may have trouble swimming if it's a lot of fin missing. The tattered sections will probably turn a fuzzy sort of white after injury, not long fungal strands, just discoloration which is normal. If fuzz grows, then fungus has set in. Once the fins are healing, the scared area will probably turn black along the fin edges and injured areas. It's kind of like a scab and will go away over time.

It's up to you whether you separate them or not. If you don't, the injury could be worsened. If you do, then you have an extra tank and work to worry about. Temporary separation probably won't do much good especially if it's a spawning issue as when they are put back together, the male would be even more excited after a separation. Once fall comes, interest should wane. I'm not sure if desire for sex or eggs was involved with your fish but it is a common cause for fin ripping among goldfish. Sometimes though, some goldfish are just more aggressive than others. This is one reason that fancy goldfish are often kept separate from larger, normal-finned goldfish.

I hope your fish recover and do well! I'm sure they'd love a larger tank too!

I was thinking of adding a ZooMed 501 canister filter to my goldfish tank. Could the intake on the filter harm my Comets? Is any filter a risk?

See, the thing is, if you are talking about healthy fish.... they shouldn't get stuck to the intake tube on a filter. I have some pretty lazy and slow goldfish in my aquariums but when it comes to the filters intake tube, they know when they are getting close to it... and as I said, healthy fish should jerk away from it in time without any tail damage.

(So if you see this happening, more than likely you have an ill, weak fish.)

My fish are healthy, at least as far as I can tell. The reason I asked is because I never saw any of my fish, tropical or otherwise, having problems with filters.

But it is certainly good to know.

Yeah, no, that is a GOOD thing if they dont have trouble with the filter... it means they are perfectly healthy! :D

(at least from what we can tell)