I am sure there are a lot of variations. Mine, very aggressive and active during the primary growth phase (9" to 16" in 6 months). Since the growth slow down she is less active and more prone to hanging at the back of the tank.
Filtration is a Emperor 400 Penguin 200 and a Jebo 839 canister (rated for 100 galons). WC regiment = ~40% once a week. Tankmates can be seen in my sig.
Water params are always straight: 0 ammonia and 0 nitrite ~20 nitrate
What size is the tank? Otherwise it looks like nothing chem is the problem....It's probably just a shy Poly, you can try a stakeout to see if he's active at night
I would actually say less often....Also do you have a background? is it on the back or the sides as well also (even though it's probably a waste) you could try dither fish they'd have to be rather large however to avoid predation from your other polys
Generally speaking I would consider weeksi as shy and not very active fish. My weeksi will come out ot eat during feeding time. If you're bored you can send him my way.