Are your sumps standing directly upon the basement floor? If so, then perhaps build a short stand to elevate them a foot or so, and then drill the bottom in one or more places and install drains with valves. You could open these and drain a few gallons of water every so often, hopefully taking the settled detritus/mulm with it.
I'm wondering if the current push towards bigger/faster/more/more/more has reached a tipping point, beyond which the benefits are outweighed by the difficulties attendant with all this gear. You have 500+ gallons of sump capacity, which sounds great...but now are finding it awkward to properly clean these monstrous sumps, largely because of their sheer size. One of the advantages of big sumps, IMHO, is all the space that they provide for media, heaters, etc. while still allowing clear and unrestricted access to all that stuff...and of course they increase the overall water volume in the system which is never a bad thing. But if the sump gets so big that you can't reach all of it easily...or if you have so much biomedia that it restricts access...then where's the benefit?
I haven't re-read all this or the other thread, but I seem to recall that at one time you were convinced this was not a bacterial bloom, and now sound as though you are re-thinking that. I also recall someone, think it was
esoxlucius
, warning of possible accumulation of detritus as being a possible factor here, but don't know if you did much consideration along those lines.
I don't mean to offend by my comments. I am definitely a simpler-is-better kind of guy, and I get the impression that you tend more towards the throw-money-at-it, more-is-better school of thought. Both of them certainly have merit; if your current approach isn't working, maybe try swinging in the opposite direction for a bit? Maybe a bit of a livestock reduction would help?
I think you said before that this is supposed to be enjoyable. I couldn't agree more; why bother otherwise?