I have a fluval flex 9 gallon that has been set up for about 2 years. I used to just have shrimp and snails but about a year ago I added a betta. In the last few months every shrimp or nerite snail I put in dies instantly. I checked the water. Ammonia and nitrates are 0 and nitrates are 15. PH is 6.7ish. Today I bought 4 nerite snails and acclimated them and put them in the tank. They haven't moved yet. I have no idea what's going on.