While researching the best DI resins to use in an RO system, I came across some interesting discussions. Below are some things I found for reference. They are snippets out of discussions on this topic so will not read like an article. The links to the source are at the bottom of the page.
** DI resin starts releasing weakly ionized substances even before it is exhausted. Some of these include phosphates, silicates and nitrates. Additionally some of the weakly ionized substances do not register well on a hobbyist grade TDS meter so levels are probably higher than you think.
Try some Spectrapure SilicaBuster DI resin, its mixed for our specific water conditions and will outlast any other by at least 30%.
Best of all its on sale:
DI life is dependent on several things and no
ones are the same. First is how well the membrane is working or its rejection
rate, the better it is working the lower the TDS will be going in to the DI.
Next is the presence of CO2 in the water, if it is present at higher than
normal levels it eats DI like crazy.
Another is the quality or the resin, nuclear grade, semiconductor grade etc.,
how fresh it is as it has about a 6 month shelf life in the best of conditions,
how well the cartridge is packed and how it is configured as in vertical with
bottom up flow which is the best or horizontal which is the worst.
Best case you can roughly figure 3,000 to 6,000 total TDS per pound of fresh resin.
So if the water entering the DI has a TDS of 10 you could estimate to get 300
to 600 gallons out of a pound of resin. Again that is best case with normal
mixed bed DI resins. In my case it can be as little as 150 gallons with normal
bulk nuclear grade resin due to high CO2 to over 1,000 gallons per cartridge
with the MaxCap and around 3,000 gallons out of a SilicaBuster is it is
preceded by a MaxCap. I have tried every resin available and those work best
** Color changing resins can be very unreliable at best. They often change is streaks or stratify so you don’t get a good visual. Usually the color change is either incomplete or too late and you are passing TDS into the treated water. Also color changing resins use a dye which adds TDS and may not be desirable.
Nothing is a replacement for a $25 TDS meter.
** Yes, I have the TDS to show actual readings, i don’t just go by the color change but it is a good visual indicator. Never heard of the resin giving off anything unless it is at the saturation point.
** I just got off the phone with Purolite. A big manufacture of DI resin. She asked some questions about how its used in our applications etc. And explained they would not be responsible for the death of any fish due to using color change resin. She said you cannot have color change and food grade in the same resin. The typical “nuclear grade” color change resin is LAB grade not FOOD grade. She said LAB grade is NOT safe to drink.
Thoughts??? Everyone (it seems) uses color change resin, but this woman made it sound as though food grade would be better applied. She could not tell me what it is about lab grade color change resin that causes problems (or if there is a problem at all) but she said it had to do with the dyes used in the color change resin.
Anyone have any input? Ive been drinking color change resin water for years, (yes Ive heard it can strip your body of electrilites SP?)
** Are we best using Food grade non color change resin???
f your going to drink it why do you even have it running through your DI ? Bypass DI for drinking water.
** If nuclear grade is not suitable for fish then you would have heard about this LONG before now and none of the major ro/di aquarium suppliers would be carrying it.
** I just spoke with another person that sells many different brands and styles of resin. He said “we dont recommend using color change resin for a reef tank”, I asked why and he said “where do you think the dye goes?”
Just becaues “everyone uses it” doesnt make it right.
Does anyone know the characteristics of the dye used in DI resin?
** Again, people have been using for more than 10 years, if there was an issue ( regardless if technical right or wrong ), there would have been sufficient evidence that it is indeed harmful to fish by now.
Having said that, I don’t use color changing anyways, if you have a TDS meter you will find the color appears to be exhausted before the tds actually starts to go up. Can usually run it another 30 days ( depending on usage ) before truly exhausted. So if you don’t have a TDS meter that is the better way to know when to change.
**I have suspected the dye is copper based. Greenish/blue almost always means copper content.
It may be only sligtly toxic, but just because people haven’t raised this issue before doesn’t mean there isn’t some small negative affect.
** The color change isn’t necessarily done by dissolving a dye and releasing it into the output water. The Purolite web page seems to indicate otherwise for at least one product:
It’s interesting, though, that the representative said that these resins aren’t rated for potable water.
** Yes, most are mixed bed resins, it should come premixed as like I said earlier, they are not usually 50/50 mixes. Too much of one or the other and it changes the pH of the water. Ask for a mixed bed, nuclear grade resin.
there’s 2 kinds that make up the di resin… google it.
Otherwise there’s nothing special about it, don’t let people tell ya to order it from a certain place, it’s a generic compound and cheap to produce.
Oh, and the color changing stuff is overrated and overpriced, just spend 20$ on a TDS meter and you can measure the output of the water to tell when to change it..
There are many many different blends and types of DI resin and they are very different. No, it is not usuallly a 50/50 mix of anion and cation resins, there is usually slightly more of one than the other so it does not mess with the pH of the water. 
** Check to see if your municipality has started to add chloramine to your tap water.
This will kill your membrane quickly. (accidental hot water will also)
** The MaxCap DI cartridge from Spectra Pure almost trippled my water output before needing a change. 
- The Reef Tank. “Forum Discussion” The Reef Tank, December 31 1969. Accessed – December 11 2013 <http://www.thereeftank.com/forums/f6/does-my-di-resin-need-replaced-157312.html>
- The Reef Tank. “Forum Discussion” The Reef Tank, December 31 1969. Accessed – December 11 2013 <http://www.reefcentral.com/forums/showthread.php?t=1229662>
- The Reef Tank. “Forum Discussion” The Reef Tank, . Accessed – December 11 2013 <http://www.thereeftank.com/forums/f6/which-di-resin-to-use-201083.html>
- The Reef Tank. “Forum Discussion” The Reef Tank, . Accessed – December 11 2013 <http://www.reefcentral.com/forums/showthread.php?t=1683815>