Sick Blackmoore/Over-fed? Fantail Goldfish

Question
Hello Lauren,
I have 3 goldfish, a fantail goldfish, a normal goldfish and a  blackmoore goldfish in my tank. My tank is Freshwater and approx 15-20L. I clean it every second week by replace 20-25% of the water. I have a carbon filter and 2-3 plants.
My blackmoore is sometimes is at the top of the water and more often than not, he is gasping. This usually only occurs in the afternoon, and in the morning when I feed them, he is swimming around normally. He is acting roughly the same when my normal goldfish had a bacterial infection, but I can't tell if he has that because he's obviously a dark colour.
Also, how do you tell the difference between a pregnant, over-fed, and constipated fish? My fantail goldfish is way fatter then she used to be. From what I see, she eats more than the other two, but I haven't seen her 'poo' for a while now. I have tried feeding her less food in another tank, but she will not eat in there.  I don't know what to do here either!

Any answer much appreciated,
Thank-you,
Kayla.

P.S. I do not know for sure the sex of any of my fish, I just assume by what they look like!

Answer
Hello Kayla,
First off I was told that common goldfish and fancy or blackmoore goldfish as the common goldfish will think they are ill and end up attacking them (As goldfish will attack ill fish), have you seen any chasing with them?
Also a 20 Litre tank is too small for all3 of them as commons grow HUGE and fancy's grow upto 15cm. So the gasping will be due to lack of oxygen, I noticed you said you have another tank? well i'd put the common in there.
I was told you need 20 Gallons per fish and then another 10 gallond for an additional fish, BUT I have had 7 goldfish in 25 Gallons for 9 years (the original 7), so i think your tank will be ok for 1 goldie if it's small although it may stunt it's growth and make it deformed.

Try finding a home test kit to test your water ammonia, nitrate e.c.t levels.
If you can get a bigger tank, like one for your b-day or just because you have saved =] i'd advise it.

The difference between pregnant goldies and fat goldies are that if there are anymales in the tank with the fat fish they will be constantly chasing them as this is spawning season.
If you know what a pectorial fin on a goldfish is, that will be thicker than on a male to a female and the females opening will be more vent shaped where-as males are more rounded. But blackmoores are generally fatter fish species =], you could just try adding less food in general and feed 3 times a day instead of 2, but with smaller meals. this also will improve the tanks water.
For now i'd advise buying oxygenating tablets which you can get at local petstores cheap, to add more oxygen to the tank until you can upgrade.

Good luck and keep me posted on your fish.
=]