It might be worth mentioning that in a number of countries the tap water does have a high TDS, like 300+, so often when researching "stuff" the overseas folks will talk a lot about TDS.

A lot of the fresh water folks use a lot of RO water so this is why they tend to place more emphasis on TDS IMO.

I do happen to monitor it, but like everything else I monitor it is for the purpose of comparison, when good and bad things happen.

I agree, Geoff. The context definitely has to be taken into consideration.

Echoing what Ethanyo has said, TDS readings are predominantly for marine aquariums. They are definitely important for this application, but not really for freshwater.

The TDS of your RODI water is important because it should be 0, otherwise your RO membrane or DI resin is stuffed.

So what you're really saying is that checking TDS is important for users of RO/DI filters, not necessarily marine aquarists. I never checked the TDS of my marine tank, but regularly checked my FW aquariums.

My feeling is that TDS is worth monitoring [for me] as it shows the entire mineral content of the water, including things like salts that aren't picked up when testing hardness, which have an effect on soft-water fish and how they osmoregulate. I had HITH problems with the Geophagus in my old 1400L tank with the cement background (pH ~7.5, low-moderate hardness, TDS ~150ppm) that were solved by moving them to another tank with no background and water with pH 7.2, low-moderate hardness, and TDS ~85ppm. So it is certainly possible to have low pH and hardness, while having high enough TDS to cause trouble for soft water fish.

IMO the best practice is to look at water parameters as a whole, rather than individual values. Many hobbyists (new and experienced) still seem transfixed on pH as the one and only parameter to monitor. Here's the article I mentioned before that has a good explanation of the importance of conductivity and how it translated to osmotic pressure. Starts on page 3 and is a fairly long and involved read, but worth it if you're interested in water chemistry.

http://www.tbas1.com/Exchange/The%20New ... d%2011.pdf

DI resin should last 1-2 years and RO 2-3 years.

DI resin will not last 1-2 years unless you are not using it regularly. I was using a minimum of 20ltrs a week of RODI water. I changed my DI resin every 6 months when the TDS got to 5 coming out of the RODI unit. And i have a dual stage DI unit.

It all depends on how much water you are making and how much TDS the water is that you are putting into it. Colour changing DI resin helps. starting at blue when fresh and changing to brown when depleted. My TDS going in was 120-130 depending on the time of the year. Coming out of the RO unit was anywhere between 0-30TDS, (again, depending on the time of the year). Invercargill tap water gets a silty taste in summer when the reservoirs get low and the TDS becomes even higher.

Also RO membranes only last 2-3 years when you look after them with regular back flushing.

I back flush mine for 2mins after every use and 5mins before I make any water. It can make a huge difference in how long you can use it.

DI resin will not last 1-2 years unless you are not using it regularly. I was using a minimum of 20ltrs a week of RODI water. I changed my DI resin every 6 months when the TDS got to 5 coming out of the RODI unit. And i have a dual stage DI unit.

It all depends on how much water you are making and how much TDS the water is that you are putting into it. Colour changing DI resin helps. starting at blue when fresh and changing to brown when depleted. My TDS going in was 120-130 depending on the time of the year. Coming out of the RO unit was anywhere between 0-30TDS, (again, depending on the time of the year). Invercargill tap water gets a silty taste in summer when the reservoirs get low and the TDS becomes even higher.

Also RO membranes only last 2-3 years when you look after them with regular back flushing.

I back flush mine for 2mins after every use and 5mins before I make any water. It can make a huge difference in how long you can use it.

You start with stating a fact "DI resin will not last 1-2 years" and then proceed to tell me how your situation is abnormal to the rest of us, ie high tap water tds which gets higher over summer (double what most people in NZ have).

You note there is a "silly taste" in summer, no doubt this is the addition of chemicals which will trash nearly every part of your system.

What prefilters are you using? What micron pore size are the filters?

Do you test inlet mains pressure? If you raise the pressure slightly, your DI resin will last MUCH longer, it will also increase efficiency of your membrane

What water:waste ratio are you running at? This matters a lot, and a 1:4 ratio will mostly render any benefit from backflushing nil.

How are you storing your unused DI resin? It could be that the stuff is on the way to expiry before you even put it in your system.

Also you obviously have the quality and make of the membrane and resin, if they are cheap then you can expect to get poor life out them.

If you're replacing after 6 months something is WRONG with your mains supply.

found that interesting the other week.

38-43ppm here since recent town upgrade. starting to wonder if to pure for frogs after losing half of my colony this winter.

however even when it was silty tasting it was only about 60-70 i think.

Interesting video, totally douchey presenter though.

40ppm is still chocked full of minerals though, can't imagine it does any harm. If you were especially worried you could leave your water in the sun for a week until some if it had evaporated to increase the mineral concentration (or just boil some off, but this is time consuming and costs money).