Well, you kind of have me stumped. So, here we go:

Your hydrometers - are they the "swinging arm" type, or the floating kind with a long thin top of glass. If they are swinging arms, please throw them away.

Tap water: Please. Just use RO/DI water. Yeah, I've used tap water for years - about 30 years. 5 years now with RO/DI and MUCH better results. Sometimes tap water is just fine, sometimes it isn't. With BIG water changes, it just isn't worth the risk. Sometimes chlorine dissapates, sometimes it doesn't. RO/DI water is cheap. Fish/inverts aren't.