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One of those reactors should be good for up to 20g if it's working properly the bubbles go all the way up the diffuser ladder without being hindered. I think the rate is 8 bubbles per minute.
Wait it out for a few days... the bubbles will consistently go to the top of the ladder once it has built up a little slime coat. Mine did the same thing but it works fine now. It took a couple of days.