Hi sounds like a heat problem to me? Best idea is to do as you already are doing{turning the lights of in the hottest part of the day},put in more air if you can {from a spare air pump?} put a small table desk fan on it during the hottest part of the day.
In my opinion i would be 'cautious' of such large cool water changes?
If you have an external filter - this could be placed in a bucket and filled with cold water and ice cubes topped up regulary.
Also on the bright side?The weather is ment to be cooling down by the weekend?
I also lost a scooter blenny from lack of food,one day it looked great next half starved next day dead
Good luck Matt
Hi thought about time i updated this!
Tank no 1 = 3 retic rays,1aro,2 ompuk cats,8 angels,1 syd greshoffi{19 years old},1 syd unkown,1 stripped dora,1 plec,1 silver dat,1 pim and a soribum lima!
Tank no2 = 6 auralis barbs,2 needle gars,1 molly,1 dat microlepis,1 trye track eel and 8 bristlenoses!
Tank no 3 = 2 goldfish{on holiday}1 flag tail thingy{over twenty years old},4 silver dats,1 pim and a plec!