Hi Roxy,
WELCOME to the forum. 

Keeping your Black Moor and your Common together isnr "wrong". Although it is not recommended to keep slow and fast swimmers in one AQ, your ones are used to each other - I assume - from the very start? And as you said, they get their fair share of food.
Can you post some pics of your two buddies, please? And what are their names?

According to the tank size: 13 litres is far too small.    You should have min 10g = about 32-34l per fish.
If your goldies are babies now, they will outgrow this tank pretty quick and you will face problems. Lots of threads about this on this forum.
What kind of equipment do you have with your tank?

According to your tank I advise to get a bigger one asap. Motto: the bigger, the better... you may want to have a couple more goldies eventually...? Bigger tanks are easier to maintain too. 

Hi Roxy,

I'd say they play with each other. Goldfish are very sociable fish.

Accordingto tanks you may have a squiz on ebay...

Where do you live?

Hey Andrea, thanks for the help.

I didnt mean get 3 fish though, I ment that if Pepi was hurting Bonkers then I would give Pepi to a friend of mine and rather just keep Bonkers and get another black moor goldfish for Bonkers.

I understand that a 13Litre tank is too small but may I ask why? what happens to the fish in a small tank?

as I said I am confused because there is a lady here that has had 3 blackmoor goldfish in a 14Litre tank and they have lived in this tank for about 12 years and there is nothing wrong with them.

I am not a collector of goldfish and I wont be getting any more to add to what I have.

Hanna I live in Knysna.

Hanna explained it quite well Goldfish are extremely messy and are also very active swimmers. So not only do you need more room for the waste to sort of spread but more good bacteria to grow and break down the waste. But they need lots of room to swim around in especially your comet! Comets are like the grey hounds of goldfish let's say You wouldn't want to keep a grey hound in a small backyard and never take it for longs walks and runs. Now of course you can't take a goldfish for a walk so you need to offer this space for it at all times

Many people who have owned tropical fish in the past just don't understand this but goldfish are different to them. My boyfriend even laughed when I said how much room a goldfish needs as he is used to the much smaller space that tropical fish need. Just remember that are different.

Also learn about the nitrogen cycle if you don't already. Don't be afraid to ask any questions.

Oh and one last thing there is always going to be exceptions, I'm actually one of those exceptions. I currently own a 22 L tank which holds one comet. She has been in smaller tanks with other fish and never anything bigger than 22L yet she is fours year old this year and living a happy life. However, it is not an ideal situation and I want to give my fish the best life possible as I'm sure you want to aswell. A bigger tank also means much less work for you and a smaller risk of diseases spreading as there is more water for them to be diluted in and therefore weakened.

Ehom, 10 US gallons is equivalent to 37.9 liters, so it's more appropriate to get 40 liters per fish. Considering that the common fish will benefit from the extra space. Therefore, I would buy myself a 100 liter for these two fish But at the same time, I would be observing them closely to be sure that both are eating enough etc.