The 40g is about 3 and a half months old now.Yup, source water changes is the first thing to come to mind.
SAE eat a little hair algae but not Otos or plecos so much. Amano shrimp are pretty good with hair algae though and highly entertaining with small bio-loads. CO2 will help by keeping your plants healthier, outcompeting the algae for nutrients but that still requires the proper balance and something is out of balance right now.
How old is the 40B?
I had two amano shrimp in here for the longest time but i haven't seen them in a while, they just kinda vanished, just like one of the guppies i had in there. They got big enough to where none of the fish in there could eat them. My only guess is they died somewhere and my synodontis cats ate them before i could find any corpses. Or the cats ate them while they were alive i'm not sure.
I also went to test both my tank water and tap for phosphates and my tank had less than the tap did, which was zero. so i'm kind of at a loss on what is causing this. I set my timer to lower the time the lights are on from 11 to 9 hours cause that's the only other thing i can think of. Although my sister's tank's lights are on for many many hours longer than any of mine.