Thanks for the responses!!!
Despite dreading the maintenance I did try to do it at least every two weeks (hence all the filtration). For the last little while I thought I was over a hump or something as the level of agitation did not seem to be as high for the last few months, however if I was able to vacuum and change without too much incident there was a huge sigh of relief.
Needless to say the lady I got the tank from had no idea what was in it, it was her husbands hobby and when he passed away she wanted nothing to do with it. The fish were a bit smaller than they are now but I was aware of the horrible choices made in stocking the tank, but it was what it was.
I suspect the larger fish is the ID and I cannot fathom why those are sold for hobbyists. I have, for about year tried to find an aquarium store with a display tank that is willing to take the larger shark, but have had no success at all, at least in a reasonable radius for transport from where I live.
I was hoping that the sudden deaths were due to the larger fish freaking out for hours and that there wasn't some other issue that required intervention.
Unfortunately I don't have any water parameter info. Initially after moving the tank in the middle of winter and transferring most of the water with the fish I was testing fairly regularly but after a while I stopped as I never seemed to have any problems until Monday this week.