It's around 20 goldfish out of ~60 gallons (1 out of 3 gallons), but I don't think that's enough space either...
That's what the previous seniors left for me before they graduate, though I'm still trying to apply for another tank of similar size (but the fee is stuck because the school budget is running low and I couldn't afford it)
OP stated in the previous post that the tank in the photo is a 55 gallon. Still not great, but please read properly if you intend on chastising them at all.
Another issue comes from the fact that plecos are tropical fish (preferring warmer temperatures of 24-26c) whereas goldfish are subtropical (preferring 20c or lower), so really they shouldn't be kept in the same tank as one of them will be in conditions that aren't ideal for them.
If there are any ways that I could know that a pleco is starving besides flipping its stomach up and observing it that way, please let me know -- I hope people can realize that I don't want this to happen either.
I can swear that the bruising didn't happen until this week. As I attend school five days out of seven, I see the fish basically every day, and I couldn't notice anything wrong with them until I can see their stomach when they stuck onto the glass -- which brings me back to the question: how can I notice them attacking each other and that they're starving without manually checking their bellies?
Watch them shortly before, during and after you feed the tank. If you notice any chasing, guarding food or other behaviours that indicate one plec is keeping the other away from food, then you'll know.
Once in a while if they don't sit on the glass frequently enough for you to watch them, try to catch them (one at a time) in a net and inspect them. If the other plec is noticeably fatter than this one, then that should be a good indication. Will also give you a good scope on how the pleco is healing if at all.
General care for plecos involves:
- Plenty of driftwood/bogwood which it can suck on in the absence of any other food source.
- 22-26c temperatures (as you've already said your tank is maintained at, so you're good there).
- Caves, tall plants*, and hide-outs.
- Lots of free floor space. A good rule of thumb is that plecs should have enough space on the floor of the tank to comfortably lay out its whole body while still having space to swim back and forth.
- Algae wafers, pellets, fresh algae (if possible), various fruits and veggies such as cucumber or lettuce.
While many species of plecs are rather bold, hardy and territorial, I've found that they're easily stunned and somewhat shy. Having large caves where they can hide when there's a ruckus in the tank will provide them with safety.
*Make sure that any plants are well buried or weighed down as plecs can pull them up when moving past them.