Will the shrimp be okay without substrate? I've read a thread on this forum they will get stressed in a bare bottom tank. Mainly because its too slippery?
And babies will have nowhere to hide.
The back half of the tank is Flourite black for substrate, BB to build up, it's against the sponge so the sponge should pull some debrib from the substrate like a UGF, gives the moss a place to plant, babies to hide, stuff for them to pick at, but won't degrade like active substrate and since it has a high CEC, it may retain some of the Sosei I dose to help release it. I also seeded underneath the substrate with Bioplus, EI, Sosei and Mineral plus to get it nice and full of minerals, fulvic acid, and baby food/bacterias. The idea, to me anyways, of the half and half tank is the glass part is easy to feed, and if you notice, shrimp poop mostly when they are eating, so I feed on the bare part and can take an airline hose and suckup the food/poop so it doesn't sit there. Easy to keep it spotless.
I also have cholla wood for babies to hide, lava rock piles, almond leaves, and I am going to grow moss all the way up the HMF sponge wall as well, so babies should be fine and they do have substrate, just about 40% of the tank of it, that's all. All my lava rock has been in tanks for 1-3 years and have nice algae, bits of moss, biofilm on them. I did setup the tank quick and added shrimp in 3 days but everything I added was well seeded material.
This is a test to use a HMF filter for low cost and maintenance, no active substrate to lower pH to 5 range and keep the pH around 6.7 but keep all the minerals the same and everything else to see if they really need a low ph or can adapt to it as long as their other params are in check.