I was wondering how many Aphyosemion australe killifish I can put in a 10G planted tank?

With plenty of plants for cover I think you could maintain up to a dozen australe in a 10 without any problem. Twenty could even be done with significant weekly water changes. But after a certain point, you lose the esthetics and it just becomes a crowded "holding" tank.

Andy

Of course you could put a pair in a heavily planted tank & let nature take it's course. ;) The fewer the adults, the more fry will have a place there. But it sounds like your killie care has already helped nature take it's course.

If that 10-gallon tank has been running with fish for a while that make the transition very easy. Just don't replace killies whose fry could be confused with those of the australe. (Or whose fry might munch on the australe.)

Some will raise up a dozen pairs of australe from fry in just a 10 with weekly 50% partial water changes gravel vacuumed out of the tank & a sponge filter. Oh yes & a tight fitting hood & maybe a glass black-worm bowl for post feeding browsing.

If it is a new tank, top the gravel off with gravel netted up from a healthy established tank. Also move a sponge filter in from an established tank. Some people even run an extra filter in a tank for such an occasion. Half the set-up water could be the previously treated stuff, left open at least over night to "season." Half could be from a healthy tank.

With 13-14 hours of light & 2-3 watts/gallon, the more plants the merrier.

For me it was TFH's David Boruchowitz who brought on a "Doh!" moment when he suggested that a 6 month old aquarium, properly maintained, is the real bio-filter. A relatively modest percentage of the beneficial bacteria are in the filter as opposed to the gravel, accessories, plants & even on the tank sides. (Not much beneficial stuff actually in the water, just waste material & not so beneficial bacteria in stressful times.) Most of us had long been cultivating that before it came to mind.

I'm sure that most of the above is familiar territory to you. Maybe there is a tidbit of use. One can already imagine how glorious your australe tank will be!