I would just use tap water and transfer over some filter material from your two established tanks to seed the bio-filter in your 90g. The bacteria grow on surfaces such as filter media, decorations, even the glass. Not much free floating in the water though. You will prob need to cycle still, but it will go much faster. When I start a new tank, I seed with seasoned filter media and start with one small fish, but you could use one of the gold fish. Just keep an eye on water params until they stabilize. Or you could go fishless cycle and just add an amonia source.
Peace,
Caleb