Silicate remove RO/RI Unit

So wondering if you guys/gals could explain the different between these RO/RI units.

So a quick look on any of the main suppliers you find several different RO/DI units. They go from 4-stage simple unit to something that is 7 feet long that cost 400 bucks.

So my question is what is the difference?

Second question is I cant seem to get diatoms to go away. I am about in month 5. I tried to set back and let the diatoms consume all the silicates but it geting so bad its clumping up the sand.

So I went 3 weeks without a water change, nothing changed. Have vacuumed up the sand 4 times in the last 2 months. Still coming on strong.

So is it possible my 4-stage RO/DI unit isn't removing all the silicates? (TDS meter shows 210 in, then 0 out)
Or is my sand still leaching silicates?

Thanks!