Questions... About Possible Breeding Behaviour?

It's very difficult to get goldfish to breed in aquariums... unless it was a really big aquarium. They do breed regularly in ponds though.

You should not mix fancy round-bodied goldfish with long-bodied goldfish for many reasons... 1) the long bodied will bully the round bodied; 2) the long bodied will out compete the round bodied for food; 3) If the long bodied is a male and does get frisky, he could harass the round-bodied female to death if you do not separate them in time; 4) long bodied goldfish need much larger tanks... or better yet a pond, where round-bodied can be kept in a tank, with a couple of them in a 55G (which means you also have overstocking issues with five goldfish in a 55G... especially if some are long-bodied goldfish).

I know goldfish start off small but long bodied goldfish should grow to over a foot long in the first year or two and common goldfish can reach nearly two feet with 16" to 18" being more common. Round-bodied fancy goldfish do not grow as long... but they should reacy 6" to 8" body length... but they do get really fat so they have a much larger body mass than another 6" to 8" fish. The math actually adds up to where a single adult fancy goldfish is equal to over 500 1" goldfish as far as body mass is concerned. This is why they need to be in big tanks and have lots of water volume.

I have two fancies in a 65G tank and I have to do weekly gravel vacuums, 25-33% PWC's and filter maintenance to keep the water quality up. Going more than a week would lead to funky water and usually an algae breakout due to excessive nutrients in the water. I do have a little 3" Clown Pleco in with my goldfish to take care of any normal amounts of algae.

If you don't have a pond, you should look at re-homing the long-bodied goldfish to someone who does have a pond... unless you have room for a 6' to 8' long tank with at least 75G for each goldfish. They will even outgrow that tank in a couple of years... which is why I suggest a pond.

That would leave you with just the two fancy goldfish in the 55G which would be much better for the fish and for you.

Goldfish will breed in aquariums and ponds alike as long as they are in good health and generally around 18 months to 2 years old. If there is not something soft in the tank (breeding mop, fake or real plants, etc) for them to spawn on the female often won't release the eggs. This causes the males to constantly chase hoping to get lucky.

This behavior also happens when the fish being chased is ill or weak. Take a close look at your large female for signs of bloating or dropsy.

For 5 fancies I'd recommend at least a 75 gallon tank to keep them long term. With the pleco you are looking at a 100 gallons or larger. With goldfish the larger the tank the less problems you'll have.

If the female is getting harassed too much, you should move her or the male(s) or put in a divider or she could end up getting sick from stress. Be careful when doing PWC's in the future and make sure the incoming water temperature is close to the tank's temperature. Adding too cool of water lowers the water temp and then as it warms up, that simulates the warming waters of spring and prompts fish to go into spawning mode. Also, if your tap water baseline chemistry is a LOT different than your tank's water chemistry, then doing a PWC could prompt a spawning session.

Another thing to look for on the males are breeding tubercles (aka breeding stars) that look almost like Ich but they are mostly on the leading edges of the pectoral fins and sometimes on the gill covers and leading edge of the dorsal fin. If the males that are doing the harassing do not have these breeding tubercles, then they may not even be males and could just be bully goldfish... although most goldfish are really peaceful except for when spawning.

I would put a divider in the tank to give the female a rest if you don't have another tank for her or the frisky males. You can make a divider with egg-crate lighting material like this... http://www.professionalplastics.com/EGGCRATELOUVERS-LIGHTING It's available at most lighting stores, big box home stores, etc.