On the other side of the coin, if your source TDS is consistently that low, you could simply run it straight through DI only, ditch the RO membrane altogether. Water production may be a lot quicker in that case, although not sure if contact time going thru DI is a factor, probably is. Don't necessarily recommend you do this, but I always like to look at things from a different angle. With an oversized cartridge full of DI media, or even a number of standard DI cartridges in series, you could probably get a good amount of 0 TDS water for your tank before the DI is depleted.
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