I just bought 2 neon tetras from my LFS. One of them has a dark redness around his gill area on the left side. His other side looks fine. He's dull in color. The other tetra that came with him looks fine. The LFS had just remodeled and cleaned their tanks. Could it be ammonia burn? Gill flukes? A deformity? He kinda starts to sink backwards a bit, but then immediately rights himself up. Is this considered lethargy? There is no gasping for air, no itching.