Filtration for brackish tanks ?

Hi, I'm new here, and I'm setting up a 15 or 20 G brackish tank, and perhaps a much smaller one also, if I have more luck with shrimp hatching than I expect... and so, I hope to use the tanks for hatching shrimp larvae. Amanos, hopefully Blue Claw Whiskers, maybe even Flower shrimp, should one ever get berried.

I'd like to plant it as much as I can.. Java moss and ferns, but I am not sure about filtering or the cycling of a brackish tank. I've read I can start with media from my FW filters, and that the bacteria will adapt over a period of time, if I increase the salt content gradually over a week or two. I have sponge filters, HOB filters and corner inside filters. Which one would do best in this type of tank or should I have both ? Already have Instant Ocean and one of those floating things to measure salinity.

Obviously I'd have to cover the intake of a HOB with sponge to prevent larvae being sucked up, but that's not a big problem, and I can even cover the intake grids on the corner filters.. I use some of that dense filter felt material that's used most often for sump filter socks. I am mostly concerned with, first, cycling it properly and second, maintaining it thereafter in a reasonably stable way to give the larvae their best shot at survival. I'd love to put fish in it, but doubt there are any that would not eat the shrimp larvae, so that's too bad, but such is life.