Welcome to TFF!
The discoloration could be from driftwood, if you have any, or just a general lack of maintainance. Carbon will help with this, as will water changes. Some people run charcoal all the time, some never do. I only run it on a newly cleaned or set up tank, or to remove medications. None of my tanks run it on a regular basis, none have undergravel filters, most don't even have gravel.
What you are dealing with is a case of old tank syndrome. Do not do a large water change, the difference in the old water to the new could cause problems, as your fish are acclimated to the old water. I wouldn't do more than 10% daily, this will allow your fish to become used to the new water conditions.
Under gravel filters are a cheap, but realyt dirty way to filter a tank. On well maintained tanks the underneath of the undergravel plates looks like a mud puddle after a year, that is all rotten food & fish waste. The only way to properly clean them is to tear down the tank yearly.
Plecs are really messy fish, an undergravel filter is really not suited for them. A good hob filter is better, you didn't state your tank size, you want to turn over your water a good 5-7 times per hour with those plecs. That means a filter that filters 250-350 gallons per hour.
Anaerobic bacteria do not suck the oxygen out of the water. Anaerobic means without oxygen, aerobic means with oxygen. Anaerobic bacteria will produce pockets of hydrogen sulfide gas, it smells like rotten eggs, & is toxic to fish. If you have any larger rocks or other decorations, there is probably anaerobic bacteria present. You can tell by a darkening of the gravel under these objects.
I don't usually suggest chemicals for tanks other than dechlorinator, but in your case, I would treat with ammo-lock before a gravel clean as well as after. This will prevent an ammo & nitrite spike from happening when you start to vac & clean the bottom. There is probably a ton of gunk built up, you will be releasing some of this into the water no matter how careful you are. The nitrobacters in your bio-filtration will be temporarorly overwelmed, causing a spike. This is a trick I learned years back from a shop owner who goes on service calls, he's been doing this since the '70s.
Congrats on getting back into the hobby, this is a good place for answers to most any question.
Tolak