Hello there, we need some advise/help please? We have 3 plecos in out 150g cichlid tank. The ammonia is 0, nitrites 0 and our nitrates about 30-40. pH is about 7. Tank has all mopani wood & african "wood/root". Tank had been established for 9 yrs. we do 1/4 water changes about every 4-6 weeks. Filtered by 2 magnum 350's. Also have 2-3 clay pots. There's approx 30 cichlids (some are babies born in the tank 1 to 1.5 inches). We also have 2 synodontis and 2 raphael cats. Noticed a few weeks before our last gravel vacuum one of the babies were harassing one of the plecos. At the base of his dorsal fin there was a small (little bigger than a sesame seed) white spot. Flat not raised nor fluffy. The baby kept hovering over the Pleco and kept "nibbling" on him. Now 2 of the plecos have identical spots on both sides of their dorsal fins. Saw the baby nibbling again. The nibbling baby was relocated to another tank. The 3rd pleco has a tiny spot on one side of his dorsal fin. Plecos are eating normally & behaving normally. They're fed every other day. Some flake, some shrimp pellets, some pleco algae wafers, some frozen brine shrimp and some krill occasionally. The diet is varied from these every other day they get as much as they can inhale in 5-10 mins. I've never seen this on any of the fish. All other fish are well and some breeding. All are vigorously eating and lounging normally. I'm going to try to attach some pics. I have searched the threads and didn't see anything that was close to this issue; however, Om new and may've overlooked it or not searched in the right place. Thank you so much for your advise & input 
