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I always mix my own.
To me it's a matter of a variety of factors.
1) Premixed is more expensive
2) I'd have to lug the containers all the way to my apartment from the parking lot (gimme a break, I'm old
)
3) The salinity may not match my tank (they don't mix it exactly the same every time at my LFS, there's just a sign saying what today's SG is)
4) I have to assume that LFS is properly maintaining their RODI machine
In mixing my own, none of the above is a concern.
Downside of mixing your own is that
1) RODI machines are an initial hit, and some are quite expensive
2) Cartridge replacements are also not cheap (my approximate maintenance costs are $200/year for 5 cartridges)
3) it takes ~ 5 hrs for me to get 5 gals of RODI (my machine does 25gpd)
4) I have to have a bucket w/ a powerhead and heater in it churning away for 24hrs before I do a water change.
For me, my own RODI was the logical choice. I'd have the RO, carbon and sediment filters anyway, because I live in an area where the water is bad. Having the aquarium therefore only adds the DI cartridges to my expenses (~ $60/year)
