Parts
Model number to replacement part: which cartridge, membrane or lamp fits, on what interval, and which cross-fit alternatives exist.
You already own the equipment. These pages give the exact replacement part number, the interval it is changed on, and the alternatives that fit the same housing.
Express Water WH300SCKS Replacement Filters: Which Cartridges Fit, and Cross-Fit Alternatives
Express Water WH300SCKS takes FLTWH2045S1, FLTWH2045K and FLTWH2045C1 (set FLTWH2045CKS1), 4.5 x 20 in, changed every 6-12 months or 100,000 gallons.
iSpring WGB32B Replacement Filters: Which Cartridges Fit, and Cross-Fit Alternatives
The iSpring WGB32B takes one FP25B and two FC25B 20 x 4.5 in cartridges, changed every 6-12 months or 100,000 gallons. Pentek DGD and EP-20BB cross-fit.