Potentially ill Peppermint... tank transfer... tank mates.

Hello, I have a few questions I'd like to ask if that's ok. xD

I haven't been on the forums for a while due to everything that is going on at home.

My tank is a Jewel Vision 180 (?), a bow front 180l tank. I have the main pump running along with an extra which is fairly strong. The filtration is strong and is not the problem.

My tank is full of Corydoras, a Male Peppermint Pleco, a Snowball Pleco, a Flash Pleco, some kuhli loaches as well as some mid water fish.
Snowball Plecos are known for getting fairly big while mine has stayed quite small which has made me think that it may be down to the species that I have got. Flash is going to stay small as you most likely know and Peppermint will most likely hit about 6 inches.

Peppermint has chosen a cave while flash uses the dark section under the wood in the middle as his cave. Snowball sometimes shares the wood and sometimes decides that she wants to chill elsewhere. Territories seem to be ok and there is a cave that is unused by plecos at the moment, the corys and loaches seem to enjoy it though.

The issue is my sister's peppermint. She has a small tank (not sure of the size, about 12 inches across) which I do not believe to have efficient filtration and does not have a big enough cave for her pleco. This peppermint is lacking in it's growth and has got a very clear pattern that it seems to have where half of it's body is nearly pure white. I have seen my peppermint go towards this pattern in the past where he was stressed but never as clear. I am wondering it the pleco has stunted growth due to the size of the tank and has falling ill due to the way that it is setup? What I have done for now is moved it into my tank where it will have more space and will have a cave available to it.

So, I have three main questions;

1) What do you think the issue could be? (Bearing in mind that the parameters are stable as far as I know).
2) Do you think the tank may be overstocked with another peppermint added?
3) Do you think there will be a big issue with my male peppermint?

I'm going to keep a close eye on them but would appreciate your opinions.

EDIT: Just an update, my sister's peppermint has also decided to move under the wood where we now have a peppermint, a snowball pleco and a flash pleco living together. Her peppermint has got normal colouring right now so I believe that it was the stress.

yep, stress. Tell your sister that a peppermint needs at least 60ltrs and good filtration, including some form of current. if she wants the fish back she will have to sort it out.

I have told her, unfortunately, we do not have the space for another fish tank so it may need to stay in my tank for now. I am leaning towards believing that it is a female more and more as my peppermint seems to have accepted it into the tank.

I have not said much to her about the fish that she has been buying as it is none of my business, but I think I may have to now. More research needs to be done before she buys her fish.

If her peppermint does turn out to be male and my peppermint decides to bully a bit, should I find a new tank for her peppermint or will they sort out their own territories? There's enough caves and places for them to claim so there shouldn't be a problem with them splitting the tank into territories.

peppermints are pretty peaceful towards each other unless you have two males and only one good hiding place.