Agree with Millingu
6 as adults would be pushing it.
And not only the cramping, but also for water quality.
Even with 6, as adults, you'll be needing to do every other day 50 % water changes, to keep nitrate at a tolerable level
Agree with the other comments. Smaller to medium size haps would probably be fine, but at those sizes 10 or more are a little too much for that tank imo.
Thank you all for all opinions. I’ve made my mind and decide to get a new tank.
I’ve ordered today a custom build ( in my place used market of huge tanks do not exist) aquarium. It will be 200cm/61cm/61cm (79, 24, 24 inch) and will hold almost 200gallons. Wow I cant wait?
my current 120g will become a mansion for my huge Jack Dempsey (now lives in 50g)