That ammonia level will be harming them, but the bigger problem is going to be the Nitrites that will result shortly. Stop all feeding off the fish until you get your water under control, or they will all be very dead, very quickly.
If you can do a graduall water change, thats what I'd be doing. Have a hose running into the sump tank slowly, while the pump is running, letting it gradually overflow, or siphon some out onto grass etc. Doing sudden large water changes upsets systems, and should only be used as a last resort.
If you have not already added salt, I suggest you do. You will need 3 or 4 kg of salt, add this to the sump tank after changing some water out. Standard pool salt, either lake harvested or ocean. Do not use table salt. Salt will stop the fish from being killed by nitrite so quickly, and will help them during this stressful period.
You have a pretty high fish load for a brand new system, so very little food for the next 3 or 4 weeks is going to be the key. The fish dont need much to survive.
Add as much aeration to the tank as you can afford.
Good Luck
Backyard Aquaponics
