The membrane needs to stay wet. It's probably shot from drying out in storage. Without the membrane doing the heavy work of removing most of the tds from the water, you will use up di very fast. Di is just meant to soak up the very tiny contaminants that fit through the membrane, it polishes the water.
Starting from the beginning you have:
Sediment filter to remove large chunks that would clog or damage later filters
Carbon filter to soak up smaller things and chlorine that will damage the membrane
Membrane to only allow almost pure water through
Di to polish
The first two protect the membrane and the last one finishes the job. So each part need to be in working order.
The sediment filter needs replaced when the valve on your unit shows a 10% or so drop in pressure from where it is without the sediment filter on (because it gets clogged up). The carbon when chlorine gets past it, buy the test strips from spectrapure when you get a membrane and use them on the waste water to check that. The membrane is done when the tds before the di starts to raise, you test the tap and the pre-di water for tds and if the membrane is not getting rid of like 96% of the tds you need a new one. Di gets replaced as soon as the produced water is not zero.
Or you can just put on new sediment and carbon every 6 months. That's easier but you can get a little more use out of them if you test it. Once you get the membrane in order it will last a couple years.
There are reefers who use tap water with good results but to suggest it to someone when we don't know anything about their water source is terrible advice. 22 tds is totally unacceptable, not only because you don't know what those tds might be, but also because tds is only part of the picture. There are plenty of contaminants that don't register as tds. All that tds measures is how well each part of your unit is working, as explained above. If it is zero the water is clean, if it isn't the water is not pure. It's like a yes/no test, rather than a measure of water quality. It is inaccurate to say that a little bit of tds is safe.
