Your Ph is fine and stable ph is better than messing about with it.
an ammonia reading like yours is highly toxic to fish and you need to do a massive water change asap
Your fish is stunted due to poor water conditions as an Oscar 18 months old should be a lot bigger than 8", in fact should be near 12".
you are under filtered on the big tank I would say, you want a flow rate of 750 gallons an hour for a 75 gallon tank with HOB filtration.
with the Oscar in the ten gallon you need to change 80% of the water daily as an 8" fish will be producing a lot of waste, with the 75 gallon you need to do about the same weekly, 25% fortnightly is no where near enough.
This article will help you viewtopic.php?f=26&t=13651
Do you change the carbon and if so how often?
sorry for all the questions but we need it to help