So, here's the thing. Since you add baking soda and crushed coral to your snail tank, it's safe to assume that it's higher in KH. The other tank could low(or none) in KH or just lower than the snail tank. Depending on where you are, you could have absolutely no KH. My API test changes color on the first drop, meaning, without any additions, I have no KH.
Suspect your test kit before your start dumping a lot of stuff into your tank. It's also possible something is in your other tank(not the snail tank) that's affecting KH. Have any suspicious rocks?
Suspect your test kit before your start dumping a lot of stuff into your tank. It's also possible something is in your other tank(not the snail tank) that's affecting KH. Have any suspicious rocks?