Gallons per hour are 270 as stated on the box, which realistically is reduced when adding age, filter media and muck clogging it up (as in, not in a perfect lab water measuring environment).
29x4 =116 average gph necessary for a 29G tank - high filtration would be 10x turnover 290 gph (really less because the substrate and stones take up a little volume,
)
So the point in addition to the gph is what media you have in the filter, and of course it being well cycled.
The stock/standard new media from a new filter set up will take somewhat less time than when a fishless cycle is done and you can help it by sharing the media of a sound/stable tank.
Squeezing in a bunch of muck from the other filter into the midst of the new filter media.
And you note having a sponge filter as well. Additional filtration, is it already cycled?
Then the question of what kind of "12 more fish" - big Goldfish or Oscars or Gouramis, no (lol), but Cories would be good, and rounding out the other fish groups.
A 29 tall tank does limit the wide /horizontal swimming of the shoaling Tetras and Rasboras. And a 29G long would be even better, (or a 40G breeder with a great big wide floor space). But that could be a topic for a later upgrade.
