It doesnt matter.
Prime works pretty much instantly.
I put the stability directly on the main surfaces I want it to
colonize...the sponge of a sponge filter...ceramic media...wherever I want it to grow, i apply with a syringe that has a 14 or 16 guage needle. You can get those at a TSC or Rural...
I did a lot of Walstad aquariums and jars. Without some flow...it is considerably harder. I always put at least an airpump hose in...doesnt neccesarily have to have a stone.
You arent replicating nature by not having any flow...even lakes and ponds get currents. Sometimes by springs, sometimes...
Incas can be really fast when they feel like it.
I get more comments on the Inca snail in my work aquarium than I get for the Endlers.
The snails might like a boiled/blanched vegetable or two occasionally.
Looks really good. I have a gold inca just like the one you show in the photo. Ive never had an escape attempt but theres a lot of current right at the surface so I think they avoid it.
Your bettas look healthy and happy.
They do much better in groups. They arent neccesarily a schooling fish like a clown loach or tetra, but they like having a social hierarchy with conspecifics. I have had solo endlers literally swim themselves to death circling the perimeter of the aquarium.
For a single betta a giant pickle jar, lexan or tupperware tub, extra large vase--- anything will do for a short stay for medications. Or you could flip that and put your filtration in the extra vessel and oeave the fish in its aquarium with a bubbler,
I would move him to a separate aquarium or bowl temporarily and start treatment. If his fins are deteriorating and you can see it happening... things will go downhill pretty fast. Bear in mind the fin rot and stress from it can cause something else to start- such as velvet or ich.
Fin rot can be bacterial or fungal...usually fungal fin rot winds up being white or pale and much more even around the fins. Bacterial rot results in more ragged fins and unevenly spread infection.
I honestly cant tell if your fish has fin rot...if it does you can either use an over the counter...