You need to test your own water, you may not know that as your water evaporates the salt doesn't so ever since you've had the tank the salinity is rising. You need to top of regularly with ro
(plain, unsalted) water. This may be your problem. I top off twice a week on my 55 and 4-5 times a week on my ten.
Buy a refractometer off eBay and get that in check before purchasing any other livestock...