Please remember, the sediment filter and carbon block have very little to absolutely nothing to do with TDS, the yare there to protect the RO membrane from sediment, particulates, colloidal materials and chlorine. The former are measured in microns and up to visible with the human eye or about 40 microns down to maybe 1 micron or in some cases 0.5 microns. The latter, chlorie is usully present at around 1 ppm or mg/.L so would be 1 TDS.
TDS or dissolved solids are just that disslved in the liquid not suspended and in the 0.0001 micron range, many many times smaller.
The RO membrane is the most expensive piece of the system and the workhorse, removing 90 to 98% of the TDS, it makes no sense to not change filters when their sole purpose is to protecdt it so it can do its job efficiently. Dirty filters reduce pressure to the membrane making it less efficient and reducing DI life if you have DI, costing you money and lower water quality. Your reef suffers.
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