Pleco help

Hi, I have a 30 gallon talk for a 3-4 inch long pleco named Mr. Suck. I do a 50% partial change of his water weekly, and when I do I add Stress Zyme, Quick Start and Ammo Lock. He gets a white underbelly that comes and goes along with lighter stripes on his body that also come and go. I've checked the ammonia in the tank, it's at a good level. I know he isn't supposed to be this pale but I just can't figure it out. Does anyone know what's wrong with my poor boy? I've attached pictures for reference.

Hi and welcome to the forum

Any idea what species of catfish it is?
Most plecos can grow to over 12 inches and need a fairly large tank.

Does it change colour in the morning straight after you turn the tank light on, or after water changes?
If yes, then it's stress colouration.

--------------------
A tank that is too bright will also cause it. Most catfish are nocturnal and tend to hide during the day. If they can't hide, they stress and fade in colour.

Posting a picture of the entire tank might help show issues that could be addressed.

Make sure the fish has some driftwood to chew on (it helps their digestion) and have the light on for up to 16 hours a day to encourage algae to grow on the glass and ornaments. You might not need 16 hours a day and might get good algae growth with 10 hours of light per day. But make sure there is some algae on the glass for the fish to graze on.

--------------------
TURNING LIGHTS ON AND OFF
Stress from tank lights coming on when the room is dark can be an issue. Fish don't have eyelids and don't tolerate going from complete dark to bright light (or vice versa) instantly.

In the morning open the curtains or turn the room light on at least 30 minutes (or more) before turning the tank light on. This will reduce the stress on the fish and they won't go from a dark tank to a bright tank instantly.

At night turn the room light on and then turn the tank light off. Wait at least 30 minutes (or more) before turning the room light out. This allows the fish to settle down for the night instead of going from a brightly lit tank to complete darkness instantly.

Try to have the lights on at the same time each day. Use a timer if possible.

Think it is a Gibbiceps (definitely one of the bigger plecs).

I'd rehome it cause it isn't suitable for a 30G

I have a small, bristlenose pleco who has the exact same problem, it may be caused by stress like others have said. The tank looks very small for a pleco of that size and has very few hiding spots which may cause this decolouration. However, I also read that it is completely normal and sometimes happens when they are sleeping or hungry. The tank definitely looks to small though and would benefit from a larger tank.