Hello, so I got a new betta from petco about 2 weeks ago. He was looking great until about 2 days ago he started acting a bit lethargic and not interested in food and his fins look odd. Not like fin rot but like he can't fully use his tail? He swims kinda like something hurts and he doesn't want to wave his tail and today I saw him rubbing his gills on his leaf once in a while. I don't see anything on his body that looks wrong except the tail looking like he doesn't want to use it like he cramped. His heat is 78 to 79. Tanks been going for 3 weeks, 10 gallons, he has 3 platties, a guppy, a snail, and some ghost shrimp with 4 anubias plants and a java fern. He gets spring water only for water changes with water conditioner, 1/4 tsp of salt per gallon, 2 drops of flourish for the plants, 2 drops of ph down to bring it closer to the tanks ph which is lower because of the plants. Its let sit to room temp for a few hours then I change 1/3 of the tank water and add the new water in a few cups at a time to acclimate. I tested it last week for everything at the fish store and was told ph 6.8 no ammonia no nitrite and within safe levels of nitrate. I did another 1/3 change a few days ago because he didnt eat 2 days ago but swam ok and I'm doing another one today incase something was spiking. He also gets starter bacteria dosed every few days. He has bio media in there and I'm thinking I might pick up some ammonia media too. The other fish all look healthy. I know petco had a coulimaris outbreak a few weeks ago and petsmart has ick constantly but I had quarantined the other fish for 1 week and dosing seachem polyguard to make sure they didnt bring anything. I didnt see any ick spots at the end. One platty since the beginning has a small different colored transparent patch on her tail but it hasn't moved or grown at all in 3 weeks so I'm not sure it it was just a mark. The bettas fins looked beautiful and clean until just the last 2 days so idk what happened. The other fish are swimming normally and have excellent appetites and are felled twice daily only as much as they will eat with new life spectrum pellets with garlic and fluval spirulina bug bites. No food is left over and any pellets that slip through are immediately hunted down by the shrimp. A few days ago he was swimming and blowing bubbles in his anubias so idk where I went wrong this time. Any help? I'm like 2 for 2 on petco bettas. I have another family member that has one for 4 years in a pet carrier barely changing water and I dont seem to have luck with them xD