:welcome: to AC, Dhanks!
Goldfish aren't difficult at all. For fancy types, you could aim for 10-15g per fish but it depends on the variants you hope to keep as some fancy strains grow larger than the other. It's not unheard of to hear ryukins reaching 8-10 inches so I would aim for at least 4-5 for a 90g. Ryukins are almost similar to fantails. The only difference observed is ryukins have a more pronounced hump, taller tails (there are four types identified) and taller body. Ryukins usually are more expensive than the fantails (which are a mixed selection of several variants) but some stores often sell their "fantails" as "ryukins", something you need to be aware of.
I keep four ryukins myself and they tend to be pushovers during feeding time bowling over my orandas and lionhead as they went for the gel foods and pellets. I prefer Hikari Lionhead of all the pellet brands I have reviewed. Hikari Oranda Gold also works but as it is floating, I prefer not to risk floating foods to ryukins especially as ryukins already have greatly compressed internal organs, thanks to selective breeding. Avoid foods that basically contain too much starch-based ingredients as the starch has also been known to be responsible for the floaty issues considering they produce gas when they are digested by the bacteria lining the digestive organs of the fish. Other foods you could consider are plants, mosquito larva, tubifex worms, bloodworms, krills, etc.
As for filtration, my submersible pump connecting to the filter has 6-7 times the turnover rate to the water volume. All I place in there are sponges and filter flosses to help remove the particles that cloud the water. My tank is barebottom to help make my maintenance much easier. I used gravel previously but decided to call it quits after finding it increasingly becoming a pain for me to vacuum around. Your other alternative is sand. Sand is much easier to clean as the wastes stay visible above the sand thus making it easier to clean them up.
It would be a good idea if you treat your new goldfish with praziquantel by Prazipro or Droncit as flukes are a very common occurrence with goldfish. Only microscopes can help you determine whether your fish has flukes by doing gill and skin scrapes. Fortunately, praziquantel is a very mild treatment even though it is powerful enough to destroy most parasites so it's not really a harm or loss to treat the goldfish with praziquantel, whether you suspect flukes or not. Any incoming new arrivals should be quarantined first before introducing to the 90g with the exception of the first batch of goldfish going to the main tank. Treat the newer ones with praziquantel to protect your current stocks.
Hope this helps!
