i sprinkle a bit at water changes, more for helping my livebearers remain proper electrolyte balance in my soft tap water... also add baking soda for KH and dose K+. so Ca is the only major electrolyte that I don't add in some form, and I'm thinking of starting, as my plants are showing Ca deficiency or K+ overdose, can't tell which (if there's really a difference). So short answer, yes. just a pinch or two per 30 gallons. is it necessary? probably not if you have a decent GH or fish and plants that do ok in soft water. I'm about 50/50 on both counts... fish are mix of fish that like moderately hard water (livebearers, american flagfish..) and fish that like soft water (gouramis, soft water dwarf cichlids, cory cats, dwarf plecos (clowns and bristlenoses). I really just tried it as an experiment because I was losing lots of livebearers, and lost all my celebes rainbowfish.. thought the low GH might have had something to do with it. As far as your plants are concerned... it's doubtful you'll run into any real trouble without dosing it, but it's definitely *possible* to burn through it in a high light, high CO2, EI-style setup. signs of deficiencies can be found here:
Mg deficiency from that site:
Often appears similar to iron deficiency since mg deficiency prevents a plant from properly using iron.
Yellowing of old leaves starting from edges while major veins remain green
I should also note that I dose iron pretty heavily to try to color up my crypts, ludwigias, and ceylon hygro, if i'm adding excess Mg, it's likely being assimlated during iron uptake, which I'm trying push for redder reds.
I really rambled WAY more than I meant to in that post. Hope it helps somewhat.
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