Agree it may be caused by ammonia in the water, Do a 25% waterchange and ensure the filter stays on
Feeding less and more often will help the system run better and ensue the fish have a better environment
Once you have the tank established when adding new fishjust test for ammonia rising and do a water change if necessary, nitrifying bacteria multiply pretty fast
Have added another sump to house skimmer and return pump, this sits under the bigger tank at the back, will take out the trickle filter and add an algae scrubber then turn first compartment into a refugium.
Put tank in place at the end of the 1700 litre tank which is 1.2 metres wide.
Moved everything from the small tank over, pair of Mandarins and pair of Bangai. will add more live rock next
Then add another sump onto it
Any ammonia is too much for the fish do a 50% water change at least it will halve the ammonia.I would do a 25% water change every second day untill the ammonia is 0
Shouldn't have a fish in a tank with ammonia present
2 trickle towers, undecided on what to do with them yet
Took out front overflow and plugged hole
Added another durso to back overflow, rust proofed stand and added 4x2 timber base