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Magnesium phosphate in EI macro mix?

Andy Pierce

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27 Nov 2020
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Cambridge, UK
In the APF 'macro' mix recipe for Estimative Index dosing is: 4 tsp KNO3, 1 tsp KH2PO4, 6 tsp MgSO4-7H2O made up to 500 ml in water. I always make this solution up in distilled water, so that's going to be all that's in there. My question relates to having both Mg and PO4 in the same stock solution. Can the Mg and PO4 precipitate out as Mg3(PO4)2 (like in this experiment: Magnesium phosphate precipitate - Stock Image - C030/7578 I ask because I sometimes get a "fuzzy" type of preciptate at low level in the stock bottle, which seems to have been exaggerated now that I've 'boosted' the macro mix to 3.5 tsp KNO3, 0.5 tsp KH2PO4, 8 tsp MgSO4-7H2O, 5 tsp K2SO4, so increased the Mg. Here's a picture where I've tried to focus in on a large piece with some backlighting. I haven't worried too much about it and just sort of swish it up and dump it into the tank with no apparent ill effects. It's hard to tell what happens after it goes in the tank because the powerhead blows it all around pretty directly. I notice this happening to a certain extent straight after mixing the stock solution from the dry salts added to water in the first instance, so I don't think it's fungus or mold, although it does seem to happen more as I get down towards the bottom of the bottle of mix after it's been sitting for a few months as I slowly use it up. I wouldn't have thought something could live/grow in just a salt solution with nothing else added (some CO2 from the air of course will diffuse in). Anyone else experience anything like this?

EI precipitate.jpg
 

Wookii

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I used to get some of that, and I always assumed if was a mould/fungus of some sort. I always sterlizes my bottles when making up a new mix now, and add Potassium Sorbate to my mixes now as standard, and don't tend to get the white fuzzy stuff - could just be coincidence though, as I'm using salts from sources other than APFUK now.
 

MrClockOff

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I always sterlizes my bottles when making up a new mix
Ha I’ve never thought of that! Thanks for great tip 👍

I’m using APF but I dose Mg separately. Never seen white residue in my bottles. But macro bottle is getting black from mould. Will try sterilisation next time 😬👏
 

dw1305

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Hi all,
My question relates to having both Mg and PO4 in the same stock solution. Can the Mg and PO4 precipitate out as Mg3(PO4)2...............I sometimes get a "fuzzy" type of preciptate at low level in the stock bottle, which seems to have been exaggerated now that I've 'boosted' the macro mix to 3.5 tsp KNO3, 0.5 tsp KH2PO4, 8 tsp MgSO4-7H2O, 5 tsp K2SO4, so increased the Mg.
Yes, it is definitely a possibility, all the magnesium phosphate complex compounds are insoluble in water. If you add a bit more VitC (or any other acid) it should go back into solution as they are soluble in acids.

cheers Darrel
 

Wookii

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Well I pulled some out and messed with it up close and personal, and I think it's some type of fungus. It comes out snot like, spreads out filamentous, and didn't dissolve in vinegar. I may have been overthinking the chemistry. :D

Sterilising your container with bleach and then adding the potassium sorbate when you make your mix will help.

As with most things, you can get it from Amazon:

Amazon product
That tub will last you a lifetime as you only need to add 0.4 grams per litre. I recommend you dissolve it in your mix water first before anything else, as it can be difficult to dissolve if you do it after other salts.
 

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