Can I get 3 more syno petricola or would that be too much?

Maybe not another trio of petricola. The are schooling and like friends so it won't hurt to add maybe 1 or 2 at most. You can add more if you don't made doing 25% wc instead of 10-15% a week. There very cool fish and the more the merrier there not messy so you could be fine with 3 more goodluck

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I think you would be pushing it maybe but you'll be fine. If there not dwarf synos just remember that get about 5" a piece.

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Any problems so far with them disrupting your planted tank?

I've got a quartet and I strongly believe the more the merrier for petricola. They love company!

As for can you fit more? You're running what... 1000 gph filtration, or about 600 after you factor in head height? That's turning over your tank 10 times an hour? I don't think you have much to worry about personally. And you can always do bigger water changes if an issue arises. If anything wouldn't your plants like the extra CO2?