Bob's right, this looks like a serious water quality issue. I would do an immediate 10% water change with carefully temp matched replacement water that has been conditioned with Prime, by Seachem. Add about a tablespoon of table salt to the replacement water and make sure it's completely dissolved.
You really need Prime. Prime is awesome and will help detoxify any ammonia, nitrite and nitrate in the tank. More and bigger water changes may be OK to do immediately and over the next several days, depending on the answers to the questions at the bottom of this post.
You may be having a sudden spike in your parameters. You really do need to have your own liquid test kit, API Master is excellent. This looks like ammonia to me, though I can't be positive from the pics. Ammonia and nitrite should be 0ppm, and nitrate at 20ppm or thereabouts. We really need to find out what the parameters are in the tank. Dont get test strips, they are unreliable and cost much more in the long run.
You may have "old tank syndrome", which means, among other things, that the tank has excessively high levels of dissolved organic compounds, and high levels of nitrate that over time cause difficulties for the fish due to oxygen uptake issues. There may be some finrot going on as a result as well.
If you do have "old tank syndrome", the water changes need to be no more than about 10% each time, but can be done every day.
If your nitrate levels aren't extremely high, but ammonia and/or nitrite levels are high then huge daily water changes will be helpful.
Your tank is overstocked, actually, but let's not talk about that at the moment. Your fish really need your immediate help.
The volume of the water changes that will be safe to start with will depend on the information you give us regarding these questions:
How often do you do water changes and what volume of water each time?
Do you regularly vacuum the substrate each time?
Have you recently done a very vigorous vacuum of the substrate, after having gone quite a while since the last one?
How long have the fish been in this tank?
How do you clean and maintain your filter?
What kind of pleco did you have and appx how big was he?
What temp is the tank?
What kind of conditioner do you use to remove chlorine and chloramines?
What kind of flitration, type and capacity?
What is your feeding schedule and do you vacuum/remove leftover uneaten food?
Again, not knowing the parameters of your tank and tap water I would do an immediate water change of about 10%, add about a tablespoon of salt to the replacement water and make sure it's completely dissolved. This can help if you're having a nitrite spike, especially.
It is urgent that you get Prime and you can safely use the emergency dose, which is 5 times the normal dose. (instructions are on the bottle)
I hope this helps. I do think your situation is urgent. There are goldie experts that may come along with more help, and Bob is one of our gurus here, so any advice he gives is right on the money, too.