I'm truly baffled here. I'll try to lay it all out and be as clear and brief as possible. I'm hoping someone can help.

I've got a Mansfield 160.

Noticed tank water levels dropping for no reason (approximately 2 inches overnight).

Removed tank and changed the assembly with a mansfield part. New bolts and seal. Problem persisted.

Replaced that part with an old fashioned flapper valve assembly and new flapper, bolts and seal. Problem persisted.

Bought a seal repair kit just in case the new assembly's seat had an imperfection I didn't notice. This repair kit came with a ring, some silicone and a new flapper. Still 2 inch drop overnight.

Read that there might be a suction from the fill tube if it's below the water level. Checked that and the hose is nowhere near the water.

Read that sometimes if you have the water level set too high, the water will find its own level (don't know if I believe that one, but...). I lowered the level 3 times to where now it's almost down to half a tank. Still I lose water.

Thought maybe I'll go back to the mansfield parts thinking maybe I had gotten a bad one. Nope... same problem.

No water on the floor...
No moisture around the back and under bowl...

When I put some blue toilet cleaner in the tank I saw no blue ending up in the bowl... none leaking onto the floor... yet the level still dropped.

Like I said... baffled. Someone please have an answer to this! I've spent the last 4 days at Home Depot and I'm almost at the point of just saying screw it and buying a hundred dollar toilet and installing it. Oh, and btw... house (and toilet) is only 4 years old.