I have an aquasafe unit too. Mine is about 4 years old or so and I love it. Although I have nothing to compare it with. Last year I switched to the refillable resin unit (which I can't say I'm really all that enamored with but its cheaper).
If it leaks you probably need some teflon tape where the john guest fittings (elbows) screw in. You can get teflon tape from pretty much anywhere these days and it works like a charm.
I find when you change out your DI resin you need to run about 5 gallons through the unit to clear out any nasties in the resin. I always do that, although I've never checked my TDS while I'm doing this. My canister isn't totally full either, there is some space at the top (when mounted horizontally) but I figure its all mixing around in there and my TDS is 0 so whatever.
I don't think TDS meters work in saltwater, just fresh as they usually carry low (or relatively low) levels of total dissolved solids in freshwater. With saltwater, you've just mixed in a whole lot of salt (dissolved solids), likely your meter can't read that high so it just says "error".
Hope that helps.
