Watering with planted aquarium water

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Apr 21, 2020
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Chicago, IL
I’ve read a bit on the use of aquarium water for watering orchids and the general consensus I’ve seen is that it is no substitute for fertilizer. However, what about its use as a base for fertilizer to be added to?

In a heavily planted, cycled tank, I’d assume that NPK values would be quite low since all of the plants are using it for growth.

pH drops as the nitrogen cycle takes place, so I’d assume carbonate hardness would not be an issue, not sure about magnesium and calcium content. Probably low too since the plants should be using it too?

In my own brief measurements I’ve seen the TDS of my established tanks sit right around where the TDS of my tap water sits.

Are there other considerations that I’ve missed? I’d really like to use my aquarium waste water a second time if I can.
I'd be careful using hard aquarium water for orchids as you'll have the same issues with mineral salts as you would with tap water.

Also be careful if you fertilise your planted aquarium using EI or similar fertiliser regimes - you could have higher NPK rates in the waste water than you'd ever want to water orchids.

I have a tank filled with pure RO that I do not fertilise so am happy to use that on the orchids, but my rift lake tank (filled with the RO waste) goes into a pond and then water butt to water outdoor plants and they love it.
If you have a softwater aquarium (e.g., discus), then I think it should be fine. In the winter, I water with my aquarium waste water; in the summer I add fertilizer to it to water the orchids.

Terry Root raised discus and slippers, and he had a theory that the slime the parents created for the fry helped with his slippers when he watered with the waste aquarium water. (My discus occasionally breed, but I'm not focused on breeding and raising the babies, etc)

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