I'd say about 10, Lol.
@AdoraBelle Dearheart might be able to offer her experience.
Thanks for the tag
I'd usually say to keep about ten in a ten gallon, I agree. It's not always that easy to keep to that when they're colony breeding though... lol
Lol! 10? There are twice that many in there ??. Hopefully she will chime in.
Hi! I remember you, the tank upgrade looks great!
All the plants will help, you can overstock a tank really as long as you have a lot of healthy plant growth, and consider adding extra filtration too.
It's really hard to give a solid number when they're in a colony like this, and breeding... so I'm not gonna give you a number. I have two 15.5 gallons containing guppies right now, one is the male only tank, and has a max of 9 male guppies at a time, but also has a colony of pygmy cories and three otos in there. Plus a load of plants.
My female/fry tank however... I couldn't tell you how many fish are in there right now. Three adult females, but they had about six batches of fry in quick succession, so there are probably 80 odd fish in there at this minute. But only three are adult, the rest are all fry of different ages, and the water parameters are great...
However, it's densely planted, has a canister AND a sponge filter, shrimp as clean up crew, and I'll be catching and sorting all the fish very soon because I need to sort through the fry, and move them to a grow out tank so I can feed them heavily and grow them large enough to go to the store, and reduce the bioload in the original parent tank. If I just left them all in there, eventually it would heaving with guppies and need daily water changes to prevent problems, or they would start to become stressed and succumb to sicknesses due to being overcrowded, even if I could keep the water quality up.
So... my answer is to test the water, and see how quickly nitrates rise. If they're hitting 20ppm or more between weekly water changes, you need to move to twice weekly changes, etc, to maintain water quality. If it's getting too hard to maintain the water quality, then you really need to reduce the stocking ASAP.
In your shoes, if the water is okay for now, and you can easily keep the parameters at zero ammonia and nitrites, less than 20ppm nitrates, then I'd move the one(s) with fin rot to the 3 gallon tank, and treat that. Fin rot will eventually kill them, you need to treat it. You also don't want to spread it.
Once they're recovered, sort the fish. Decide how many adults you're going to keep, I'd recommend at least 1:1 male to female, ideally, two females for each male you decide to keep. Or if you decide you just have to keep four males, pick your four favourite females, then another two females, to help spread the chasing around and prevent the females getting too stressed. That would be ten adults, plenty to keep the colony going, not too many for a ten gallon tank
Then rehome the other adults, and put all of the fry into the 3 gallon and grow them out, and home them, or see if a local fish store will take them. If you grow them out in the smaller tank, do daily water changes and feed them heavily, to grow them faster. Need daily changes because it's a small tank, you'll be feeding them heavily (several small meals a day is better than two large meals, if that's possible!) and because fry produce a hormone that stunts the growth of other fry, so daily changes dilutes that hormone and helps them grow faster.
Once you've sorted that, and your colony is starting over again, you'll be in the same boat again in a few months
The joys of guppy breeding... lol. You can continue doing this method, the same thing I'm doing, or if you can't find homes for fry, or don't want to have to keep dealing with this, rehome the females, and just keep a male only ten gallon tank. You could probably get away with 12 or so males in that tank, since it's heavily planted and you keep on top of water changes
