Overheating

Water was in it when I test drove it was fine for a
bit then temp gauge started rising to half stayed
for a bit then rose to about 3 quarters the seller
turned on heater and it went back to half the after a
bit then started rising again before we left he put in
some kind of sea lent I think and the whole time it
was water in cooling system then took off hose to
radiator filled it with water left hose off and started
it and it didnt push water out so I hear my head
gaskets are OK mabey it could be thermostat