High Nitrates readings

[Aleman]

Is your RO water really 30mg/l?? If that is the case then you need to replace the membranes and add a DI unit! If you have a DI unit already, replace the resin. If the tap and tank water contain the same amount of nitrate, then using RODI (which should be 0 mg/l) or rainwater, is the only way to reduce it, unless you put a nitrate sequestering pad in the filter of the tank, whioch will strip ALL nitrate meaning your plants will have a hard time growing.

 

[Tanksy]

Is your RO water really 30mg/l?? If that is the case then you need to replace the membranes and add a DI unit! If you have a DI unit already, replace the resin. If the tap and tank water contain the same amount of nitrate, then using RODI (which should be 0 mg/l) or rainwater, is the only way to reduce it, unless you put a nitrate sequestering pad in the filter of the tank, whioch will strip ALL nitrate meaning your plants will have a hard time growing.

Yes it's 30 mg/l. The TDS meter shows 85 ppm. Tap water has 235 ppm TDS. Mind you, I haven't added the DI unit yet and the RO system has been sitting unused but connected to my garden tap (so, full of water) for about 6 months.
Do you reckon that the membrane needs to be replaced?

 

[Tanksy]

Ok, I've added the DI unit and now TDS is 1 ppm and Nitrates 0 mg/l. I guess it's the DI resin that removes Nitrates and not the membranes.
So, here's what I'm thinking. Do a 50% WC with remineralised RODI water (using Seachem Equilibrium) and that would mean that I'd have about 20 mg/l of NO3 in my tank and maintain the mineral balance. Am I correct?

 

[Aleman]

If your RO unit has been sat for 6 monts with water in it, then it's likely that your membranes need replacing! The TDS from my membranes read 4 and 8ppm (with a source of 156, and 210 respectively). Alternatively the 85ppm you are reading in your permeate could be as a result of TDS Creep caused by the ions in the mains water crossing the membrane over time. To rule this out, run 10L of RO water to drain and see what the TDS is then, also don't forget to flush the membrane. you should see the TDS fall.

I wouldn't put 85ppm mater through my DI resin unless I wanted to exhaust it pretty quickly!

Yes if you have 40ppm of nitrate and do a 50% WC with RODI then nitrate should drop to 20ppm ... ish

 

[Tanksy]

If your RO unit has been sat for 6 monts with water in it, then it's likely that your membranes need replacing! The TDS from my membranes read 4 and 8ppm (with a source of 156, and 210 respectively). Alternatively the 85ppm you are reading in your permeate could be as a result of TDS Creep caused by the ions in the mains water crossing the membrane over time. To rule this out, run 10L of RO water to drain and see what the TDS is then, also don't forget to flush the membrane. you should see the TDS fall.

I wouldn't put 85ppm mater through my DI resin unless I wanted to exhaust it pretty quickly!

Yes if you have 40ppm of nitrate and do a 50% WC with RODI then nitrate should drop to 20ppm ... ish

I flushed the membrane when I installed it, but never flushed it for maintenance since then. Thanks for the tip!

 

[Tanksy]

The RO membrane definitely needed flushing.
The new TDS readings are
23 ppm without the DI unit
0 ppm with the DI unit

I have another question about nitrates in a planted tank.
I've read that most aquatic plants prefer to use ammonia, rather than nitrates, to live and grow. I quote:

Diana Walstad in her book Ecology of the Planted Aquarium, says that aquatic plants take up ammonium more quickly. In the case of Dwarf Water lettuce, “the turn over” time was found to be 4 hours, while nitrates turnover required a full 20 hours. Therefore, the fact that most terrestrial plants grow better with nitrates does not change the fact that for aquatic plants, nitrates are not their first choice.

Does that mean that if there's a production of ammonia (i.e. fish waste, fish food, etc.) in the tank and very low nitrates (<10ppm), plants still grow normally?