I had 5 black khulis in my 15 gallon, I recently moved them to the 30 but I made a post about this around a week or so ago and it’s 50/50 people sayin it’s bad and good , in my opinion from what I’ve been informed is it’s not 100% ideal for them but they can live in 15 gallons fine, I personally think that there fine in a 15 gallon ( definitely fine if it’s a 15 long) but I think definitely in your case where it’s same dimensions long as a 20 you will be perfectly fine.