I have had my Oscars for over seven years and I recently noticed them acting weird two weeks ago and I thought one just had a boo boo due to jumping out of the water and hitting his head on the cover but then my other Oscar started getting it and it was HITH so I treated it wish fish medicine tea tree oil and it worked so well. I'm now experiencing very high nitrate levels and I am doing 25 to 35 percent weekly water changes. I also got live plants and I changed my filter to a canister filter. I now purchased a new bigger tank for the Oscars and I don't want to put them in new water with low nitrates rapidly because I heard it causes shock after them living in high nitrates for however long. Should I put some old water into the tank. It is not a new tank. I bought it used and I'm picking it up within hours. Should I let the tank sit for 24 hours without fish? I will be using old and new filter as well as transferring decorations. But I haven't found any research on transferring fish from high nitrate waters to a new used tank. I know Oscars are hardy I just want to do it the right way.