Nerite snails being inactive for days

[Hieuvu190195]

Hello there guys, my tank has enough light, CO2, and every supplements that keeps the neon tetras, guppies, spiney-headed hillstream loaches, dwarf sucking catfish, and plants in my tank happy and healthy. I also add some salt to make sure my neon tetras can avoid fungus and so on. And I am using Seachem Excel dosing everyday to treat algae with a dosage of only 1ml a day (They died eventually yay).I change water every 5 days for 50% of my tank. 1 weeks ago I bought 3 Nerite snails (for the sake of eating algae), they were moving very fast and seemed strong, and my tank has some small snails too so I thought it would be fine if some bigger ones can survive, but I was wrong. after a night moving around, they started to fall off the glass, and landed on the sand below. I put the facing to the glass so I can observe what might happen to them, and they moved a little, then shrink to their shells. It has been a week they are being weird like that already. Some said my tank might have low pH because of CO2, but I checked and my tank is 6.5 pH, and this is said to be ideal for the fish. Some said I should put coral or shells inside for snails to have Calcium or sth, but I don't really want to mess up with what was balanced in my tank. For real I don't know what can keep them survived for 1 week without moving and eating, but until now, when I touch their shells, they still react. How can I make them move around as first day? Thank you guys in advanced, I really needed this