phospure like all DFS brands are just a rebadged name brand. Its simular to mangrove phoslock but most of the wet GFOs (phospure, phoslock, rowaphos) are simular just as most of the dry GFOs (TLF phosban, warner marine phosar, bulk reef supply) are simular to each other (warner marine and bulk reef supply IDENTICLE to each other both HC and pellet versions. only the cost is different). anyhow they ALL work effectively regardless of brand or form (wet/dry, pelletized/granulated). the difference in performance is far outweighed by the differences in cost and amount of rinsing required. the point being with GFO brand/type really isn't important especially with a reactor. I use warner marine or bulk reef supply HC simply because they are far less dusty than all others. I could care less about the high capacity because I can get the same performance by doubling the amount of non HC GFO and since HC cost nearly twice as much I dont see that as an advantage.
Oh I have the same reactor and run it like this from bottom to top: plate, pad, GFO, free space for GFO to tumble, plate, seachem matrix carbon (quality brand but more importantly sperical form is large enough not to fall thru the plate, pad firmly on top of carbon ensuring it doesn't tumble). I have a 30g with low nutrients so there's plenty of room for both and I dont change either that frequently (every two months but based on readings I probably dont even need it at all). with 75g you may be better off just using two if its too small. the HC GFO would be helpful here because you only need half as much which saves more room for carbon.