Cambridgeshire has hard, alkaline tap water. A number of iron salts such as iron phosphates and hydroxides are essentially insoluble. This can potentially cause issues with lack of iron availability for plants in an aquarium. In both the Fireplace Aquarium and the Shrimphaus, modified estimative index fertilising provides plenty of phosphates, and the alkaline tap water brings hydroxide; both can be problematic.
Chelated iron keeps iron available to plants
The usual strategy for keeping iron bioavailable is to provide chelated iron. The chelate is a molecule that can grip onto iron atoms – so long as the chelate is gripping the iron (green sphere in the illustration), the phosphates and hydroxides can’t get at it. Plants have no difficulty taking up chelated iron, so you’d hope that would be problem solved. The usual chelate, and the one that comes with the micro mix fertilisers I’m using, is EDTA. The problem is that EDTA isn’t super grippy for iron and hydroxide from alkaline water can pull the iron off, removing it from the system. EDTA works fine at neutral and slightly acid pH, but not very well at the high pH of the Shrimphaus – CO2 gas injection keeps the pH of the Fireplace Aquarium lower than the Shrimphaus. Plant growth in the Fireplace Aquarium looks fine, but I am starting to suspect the Shrimphaus needs some extra help.
Fe.EDDHSA is a strong iron chelator
There are iron chelators other than EDTA that work better at higher pH. DTPA works up to pH 7 or so, and EDDHA works up to pH 9. One potential downside to EDDHA is it has a slight pinkish colour at high concentrations. EDDHSA is more soluble than EDDHA and so claimed to act faster to relieve iron deficiencies. I picked up some 2% liquid formulation Fe.EDDHSA from Solufeed and gave it try.
Fe.EDDHSA is brown in solution
I wasn’t sure how much Fe.EDDHSA to use… per the label for hydroponics use the recommended amount is 39 ml of 2% solution per 1000 litres water giving a final concentration of 1 ppm iron*. My first try in the Fireplace Aquarium was with 0.2 ppm. To my surprise, this amount of Fe.EDDHSA is a pretty noticeable brown colour. Not an unpleasant colour actually, and in a blackwater setup this would fit in perfectly, but not really the look I was going for. I set up some cuvettes with different concentrations of Fe.EDDHSA and it looks like 0.2 ppm should be pretty colourless, but colour looking through 10 mm of a cuvette is not the same as colour looking through 243 mm worth of aquarium!
To try to get a better handle on colour I repeated the titration experiment except in a 2 litre clear plastic jug of water. Viewed this way a concentration of 0.05 ppm Fe.EDDHSA had a very noticeable brown tinge in the jug even though this level in a cuvette appeared colourless. Anything 0.01 ppm or less in the 2 litre jug setup looked essentially clear and colourless.
titration
0 ppm
0.05 ppm
0.01 ppm
0.005 ppm
0.0025 ppm
0.001 ppm
0 ppm
In the Fireplace Aquarium, which has bright white lighting on a white background, even 0.007 ppm Fe.EDDSHA had a noticeable brownish tint vs. none at all, but it’s pretty subtle. The effect is almost like adjusting the white balance of a photograph to a slightly warmer colour palette. Since the Fireplace Aquarium didn’t have any obvious signs of iron deficiency adding the minimal amount of 0.007 ppm Fe.EDDSHA provides some iron availability insurance without being too obvious.
No Fe.EDDSHA
0.007 ppm
0.013 ppm
0.02 ppm
0.027 ppm
0.033 ppm
0.04 ppm
0.047 ppm
0.053 ppm
0.06 ppm
0.067 ppm
0.073 ppm
0.08 ppm
0.087 ppm
0.093 ppm
0.1 ppm
0.02 ppm Fe.EDDSHA looks reasonably colourless
Here is the Shrimphaus trying out 0.02 ppm Fe.EDDSHA. The Shrimphaus is 260 mm front to back, so slightly more than the Fireplace Aquarium. Given a water volume of 12.5 litres, 12.5 ml of a 20 ppm Fe.EDDSHA stock dilutes 1000x to a final concentration of 0.02 ppm Fe.EDDSHA. If you squint really closely you might think there is a faintest hint of brown after the Fe.EDDSHA mixes in all the way, or maybe not. The Shrimphaus has much dimmer lighting than the Fireplace Aquarium and doesn’t have a prominent white background making the Fe.EDDSHA less noticeable.
The Shrimphaus is where having a better iron chelate than EDTA could potentially make a big difference for plant health. We’ll see in due course the effect (if any) this level of Fe.EDDSHA supplementation provides .
Notes:
* Since the specific gravity of the Solufeed Rapid is 1.32, a 1 ppm (v/v) solution is 1.32 ppm (w/v). For simplicity, I’ll use (v/v) measurements throughout i.e. where I list 20 ppm, that means (v/v) which would be 26.4 ppm (w/v).