The prefilters are only there to prolong the life of the membrane, you'd change them whenever you change the membrane.

The 5 micron sediment filter will go brown, it's totally normal and not a sign it's due for replacement, just a sign it's doing it's job.

It's common with two probe TDS meters to have one probe before the DI stage, but after the RO stage, and the other after the DI stage. That way you can monitor your membrane and your resin separately.

You'd change your membrane when it's outputting more than about 5ppm, as any higher is just going to wear out your resin and you'll be replacing it far too often, so it'd become a false economy. Replace the resin when you get more than a couple of tds from it. If you get colour change resin, it'll colour change way before it needs replacing, so ignore that and just monitor the TDS of the output.

Don't panic about 1 or 2 ppm out of the resin though, I wouldn't replace anything with 1 or 2ppm. Don't forget there are those who use raw tap water and have very successful tanks. They're probably using 300ppm tap water full of all sorts. So 1 or 2ppm is plenty pure enough and is far purer than anything you'll get from any LFS, as they neglect their units for profit.

Everything is standard size, all the fitments are the same, and dimensions. You can buy any brand. Look on Google Images, you'll see one brand's 100GPD membrane is the same as another brand's 100GPD membrane... I doubt any brand is superior, or if they are, I've absolutely no idea, it's never bothered me which brand i've got, I don't even know, whatever Vyair sold me..

On startup the TDS after the membrane is high? Are you sure you haven't got an autoflush connected up wrong? Or maybe the pressure on your unit isn't being maintained?

There's a black rubber seal on the membrane, and that needs to remain under pressure, so that it's forced outwards under the pressure, forming a permanent seal. It sounds like your unit isn't having pressure maintained on it, so while it's off the rubber seal relaxes, dirty water on the dirty side of the membrane gets through to the clean side, then when you turn your unit on, pressure forms the seal again, and the dirty water that got through is what you're measuring. That'll be knackering your resin far quicker than it should...

I'd check if it's connected up properly or if pressure is being maintained because that isn't normal...

My unit is permanently hooked up and comes on and off throuought the day with household use too, via the tap on the kitchen sink, if everytime it started up, high TDS water got through, it'd give me an above zero TDS in my conainers, but that's not the case...

@i-CONICA ...you have got me thinking now. I've been studying my RODI set up and I'm totally confused!

Its an Osmotics 4 stage pure 150gpd as per attached pic..

I have the supply (through washer tap) to 1st stage (sediment filter?).
1st stage directly onto 2nd stage (carbon?)
From 2nd stage to inlet of booster pump.
From outlet of pump to inlet of RO membrane (via pressure gauge).
From outlet (blue pipe) to DI pod.
Then from DI out to RO outlet through ball valve.
2nd outlet from RO membrane goes to drain via restrictor.
Ball valve fitted across restrictor for flushing.

If the supply (washer tap) is left open there is definitely water going to drain although not as quickly as when flush valve is open.

Now I have actually already purchased one of those auto shut off valves with 2 inlets, 2 outlets, but I'm not sure how to incorporate it with the manual flush valve etc.
I also have a high pressure switch and a float valve which I bought so that I could auto switch the pump and also add an RO reservoir at some point.

If anyone has a diagram which will show how it all goes together that would be very helpful. I had hoped the HP switch would just plug into the existing pump/transformer plugs but it just came with a couple of spade type connections????

Nice one thanks. Do you have any idea where on the water side to put the high pressure switch to knock of the pump automatically on switching off the outlet tap?

I think you might be flushing it at the wrong time mate. The flush is to clear out the water that's been sitting in it before you fill up your jerry cans, flushing it after you've filled your cans isn't going to do anything but waste a load of water as it would've already flushed out the water that was sitting there into your jerry can and be running clear.

You should flush for about 5mins before using it I think.

Yh just looked at my instructions, it's to extend the life of the membrane by removing build up when sitting, it's definitely supposed to be used before you make your water not after.
It also says the more often you run a flush the better, daily flushing is best but weekly is ok it says. All you need to do is flush it for 2-3mins each time which just lets it blast away any build up in the membrane.

Here's what it says about TDS and when to change as well. By measuring the feed water TDS and the output you can work out the rejection rate (I think the output is before the DI not after)
92% rejection is OK
95% rejection is GOOD
98% rejection is EXCELLENT

So I'd say anything under 90% rejection means time to change filters. DI is then separate like said and a few ppm on the TDS would mean time to change or whenever you choose is too much, always check your filters when reading starts getting high and then you'll know what needs doing.

Also I remember you have a tap connected to your unit I-Conica, I'm guessing you use it to wash off fish food but just incase it has a warning saying that water that has passed through a DI unit IS NOT suitable for drinking. Don't know if you knew about that but I didn't so thought I'd mention it.

Well that's alright then, I wouldn't bother afterwards though as it would've basically already of been doing it for hours so no need.