OP --
You need to get your own API liquid test kit for freshwater tanks. Post your measurements for pH, ammonia, nitrite, and nitrate.
You also need one of those tiny cheap floating thermometers for fish tanks. The old school kind with the red liquid and the numbers too small to read. Digital thermometers are all cheap and never last long. I personally have a digital and a floating thermometer.
You really need to know the temperature of the water with small heaters because they tend to malfunction often and COULD cook your fish. They also drift alot, so when you set it at 78, a month later the tank goes up to 85 for no reason.
If you have 3 common pleco's in a 30 gallon tank, I GOTTA believe they are fighting at night. You'll notice round faded spots on their skin/plates during the day if they're fighting. I had a common kill a Gold Nugget and didn't figure out they were fighting until months later--GN had several large "faded round blotches" on him and I didn't know what it was. If I'd known, I'd have re-homed the Common.
Pleco's are nocturnal, so they hide when the lights are on, and then get active when the lights go out. So most of what they do, you don't notice because you're sleeping. They could be fighting and you'll never see it.
Same thing with eating, although the Common's I had were very bold about eating with the lights on. But lots of Pleco's wont eat with the lights on, only eat at night, so it's hard to tell if they're eating or not.
The cloudy water is a bacterial bloom. It's caused by excess nutrients in the water--typically overfeeding. It's not a problem, but it's telling you there's too much uneaten food laying around, or the gravel's not clean.
The cloudy water COULD also be caused by the tank cycling. That's not good. We can't tell if that's it until you post your water parameters (ammonia, nitrite, nitrate, pH, temperature)