Newbie, What's killing my inverts?

[MommaReefer]

We have a Fluval Eco 13.5 - its a super new system, we only bought it and got it set up at the end of July 2023. Our build includes zero upgrades on the Fluval Evo system, 9 pounds of live rock, 1 inch Nature Ocean sand. We started with one hitchhiker hermit and 2 basic clown fish.

My LFS told us we could use Fritzyme Turbo Start 900 to get it ready for fish within 10 days. I had no idea what I was doing and she told me that saltwater was super easy and we'd be fine. The tank is for my 9yo... So, here I am, struggle-bussing' trying to figure out all the ins and outs of reefing so please forgive my ignorance. I cannot seem to gain information as fast as I need it.

We had a huge green hair algae problem and LFS#2 said we need a CUC asap. Their advice was 1 hermit per gallon of water, plus 2 cerith snails and 2 margarita snails. (We already 2 hermits had 2 zombie? snails) So we had 14 hermits and 6 snails. HAD!! They are dying at a rapid speed! The hermits have killed a snail and now I'm down another snail and the hermits are disappearing. We have extra shells for them... what am I doing wrong? I noticed some interesting bug-like hitchhikers, and something still yet to be fully seen, but has stringy tentacles (for lack of better words).

My water parameters are checked weekly (API Kit), with 20% water changes weekly. I buy my water premixed from my LFS.
pH- 8.0-8.2
Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 0ppm
phosphate - 0ppm
salinity - 1.030

How can I keep these little guys from dying and my 9yo from crying?

 

[Nano_Man]

Your salinity is high for years I ran at 1-023 but now I stay around 1.025 . If your tank is properly cycled then you should have a reading of nitrate. I would sort your salinity take some water out of your tank maybe a litre a day and add back with ro water and slowly get it down to 1.025 . That sounds like you have a lot of hermit crabs is there plenty of food for them in your tank they could be starving to death if not plenty to eat . But as I have said my thoughts are on salinity first . Get some algae wafers from your LFS and put it in your tank for your crabs and snails. Do you use tap water too top off the evaporation in your tank use ro if possible