Take a sample of your tap water in nitrate and let it stand for 24 hours, to increase aeration some filters you can turn up and down, or a pump and airstone, no salt then as you have some strimp and snails in there.
Aha, a couple are doing that (not the two we've been discussing) against each other, not anything else but actually one another. I thought they were fighting but no, they've begun doing this with plants too. I think this might be bacterial then. I hadn't noticed until now (been watching them for almost a solid hour now.When a fish flick and rubs his body or gills on gravel, ornaments,plants even filter.