HELP! Reverse osmosis alkality filter adding TDS

Darragh Rogers

New Member
Hi guys
I have a 7 stage reverse osmosis under sink system from ISpring
I have a TDS monitor at the exit point of the membrane. It’s measures 4-5 TDS. The nxt stage is the post Carbon which I presume removes the 4-5 TDS and then it goes to the 6th stage which is the alkality filter which adds back in calcium , magnesium sodium and potassium. Then the final stage is the UV steriliser. Now when I measure he TDS after all of this it measures close to 100 as I presume the additives have raised the TDS.
My question is - is this reef aquarium safe as all I’ve read says that the TDS should be near zero .
I’ve tried my best to find topics on this but to no avail . Can anyone give some advise ?
Thanks
 

DaveK

Well-Known Member
Your system is one designed for making really good tap water so you can drink it. That's why it's got the UV and the remineralization stages and does not have a deionization stage. This is quite a bit different from the typical RO/DI units used in the aquarium hobby. This is also why your getting a TDS reading from the final output of the water.

Now the question comes down to, is this water suitable for mixing SW for use in a reef system?

The reason we use RO/DI in SW systems is to remove nitrates, phosphates, and other unwanted materials in the water. This is why you see the deionization stage in aquarium RO/DI units. It gets out those last few things, like silicates, that RO alone does not.

I would say that the water out of your RO may be OK to use, since most of the things you don't want in the water have been removed and the items added back in should not be a major issue one you mix your SW. However, there is always a catch. It is possible that there is something unwanted getting through the RO unit that would be removed if it had a DI section. Silicates are the one thing I can thing of that may be an issue. Having them in the water can lead to brown diatom outbreaks in the tank. This is usually not too bad a problem. The bottom line, if it were me I'd use the system you have as water source initially, and then if you see a problem, consider getting an RO/DI unit for just aquarium use.

As a note DI and deionization are talking about the same thing.
 

Darragh Rogers

New Member
Thank you very much for the reply

Yeah I’m getting the brown diatom outbreaks every couple of days. Maybe that’s why

I think I’ll take your advice and get a dedicated RODI system.
 

saintsreturn

Well-Known Member
Welcome to the site and Dave hit the nail on the head here (as usual)

What you are using and measuring is an RO unit. What you want for your tank is an RODI. The RO removes a lot of variables and brings your TDS down to the readings you see int he 4-5 range. The DI resin then "scrubs" the remaining water to produce 0 TDS.

If you use your RO water, you will continue to fight the battle with algae and not achieve your goals of a "clean" system. you also run the risk of other unwanted issues. I have known people who have used an RO system and then added something like prime, but they have all eventually switched to an RODI.

Personally, i would recommend sourcing an RO chamber and use a series of valves to direct water from your RO system to this new chamber. In this chamber, add in a dual ionized DI resin and pull water from there. Test the water and it should be 0 TDS. You could use the valves to switch between RODI and RO water so you are not buying a second system just for the tank. Be aware, that this will wear out the front part of your home system faster than the back half, but should work nicely.

If you want to go with less upfront work and peace of mind, pick up an RODI system and plumb it separately for your tank and top off water. you would have two systems, but it would not strain your filters this way. I ran the 4 stage for years and my water allowed this to be more than sufficient. I upgraded to the 6 series recently just so i could control when to replace resins easier.
 
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