I have been keeping neocaridinia in a 5.5 gal successfully for going on two years now (wow, time flies). I use RODI water as I have a well with very high iron for which I use potassium salts and a product called Iron Out for my tap water.
Get yourself a TDS meter. You can buy them cheap off Amazon. I keep the TDS around 180. When it starts to creep up, I do a very small water change. I buy a product called Shrimp Mineral GH/KH+. It is a powder that dissolves quickly and completely (unlike Seachem Equillibrium which I have used in my fish-only aquariums with the RODI water). When water evaporates I add straight RODI water, tiny amounts, dripped over time.
With the hydra, I would pick them out whenever you see them. In the past, if you don’t have a dedicated new turkey baster to suck the little bastards out, then you could use a straw with your finger covering one end as you stick the other end into the water, then when you are close to the hydra, suck it up into the straw when you remove your finger. Just put the finger on the straw again when you pull it out and then discard it and rinse the straw through your tap water.
One other thing I’d like to point out is that is a lot of shrimp for such a small cube. You might have better luck with fewer shrimp, as far as keeping them alive. I must admit though, I probably have 30-50 neos in my little 5.5 gallon, in addition to several snails and some micro crabs. It didn’t start with that many, but the neos have multiplied.
For the heck of it, do you have any other botanicals in your cube? For example, in my little crusteacean tank I also have oak leaves, almond leaves, alder cones, driftwood, large rocks, inert pebble substrate and about a dozen different types of plants. This gives young shrimp plenty of grazing surfaces and hiding spots for when they molt.
Good luck with your shrimp.
I also ordered from a different person in case they had very different water levels to mine
