Those are some really nice fish you got there Henry! I used my Magnum 350 with a micron cartridge in it plus ottos to solve my diatom issue. Haven't had a problem in months!
I think another issue with your CO2 is that you don't have enough plants in there. I would also point your loclines more toward the surface to get a nice ripple. Maybe a little heavier lighting will help some as well.
You have a nice piece of manzy in there, but to me, it looks too centered. Maybe slide it to one side or the other, and use a smaller piece on the opposite side to accent it. Just my two cents.
One last thing...just keep doing your 50% WCs, and the diatoms will start to be less and less. Hope this helps...
Plants give me peace!