By adding chlorinated tap water to your system you are killing off the good bacteria in your filter that keeps the system free of toxic ammonia & nitrite.
The low bioload from 2 Oscar’s in 400 gal will show a small amount of ammonia which should convert to nitrate. But now you’re not seeing that conversion which indicates that you don’t have nitrifying bacteria anymore.
Please re-read the PM conversations I sent you mate. All the details are there, including not changing water until you have purchased the Seachem Prime.
I would say at this point the your filter is completely uncycled and you’re gonna have to start over.
Please don’t get the fy river turtle or add another ray until you get this sorted. That would be a recipe for another disaster