Issues with rain water and aquaponics

[Kobus Jooste]

I have been running all my AP (two systems) and swimming pool off rain water as the main (pool) or only (AP systems) water source and have made quite a few interesting discoveries related to water chemistry.  First up, is the complex possible pH outcomes that you can have.  All my rain water is stored in similar plastic water tanks, but I have three different roof types feeding my systems, and in the end three water chemistry characteristics emerging. 

 

First strange (or maybe not so) influence of rainwater is that it is not neutral.  Mine is pH 6.7 - 6.8, with a fery low alkalinity.  Obviously you also have almost no nutrients in the water, but as our tap water is really not nice, and we are in a serious drought, I am persisting with the rain water.  What I did not expect, is that I will have 3 different "natural set points" for my three different water bodies.  My hybrid micro system wants to be around 6.5, and if I do not buffer, it will go lower.  My large polycarb system wants to be around 7.8, thus I have to add acid to drop it down.  This unit contains duckweed, which chows ammonia and leaves the nitrates, which could explain the basic nature of my water to some degree.  The swimming pool (salt water chlorinator), even with mountains of alkalinity up additives, sits at above pH 8 if I do not hit it with acid often.

 

I have also been seeing a lot more nutrient stress in my hybrid system since it went onto rain water.  Some of it can be due to the low pH, some to the food I'm using, but a lot I think comes from the unbuffered, low base nutrient status of my rainwater.

 

Thus to conclude: After about a year on rainwater, I think it safe to say that this will work for AP, but you have to work much harder at keeping the system in balance.  You have to keep a keen eye on pH, even in a mature system, and you may have to reach for supplements when all else fails.