IMHO, there are 2 things that cause HLLE;
-Lack of trace elements ( usually caused by over-use of carbon )
-Carbon introduction to the DT
If I'm wrong, I hope someone will chime in
I'd suggest you pick up some trace elements for the HLLE, and keep your Carbon and GFO running
These 2 are readily available to me, so I linked them for you
Brightwell
Zeovit
From what I've read, you can usually run your tank with both reactors running without issue, but there are some cases/times when the trace elements can be stripped enough to show up on your fishies
The Carbon is the culprit when you get HLLE in tangs, BUT, it's usually caused by carbon entering the DT, not by the use of carbon
Make sure you
thoroughly rinse your carbon before turning your reactor on.
If this helps at all, I put my media in a bag and wash it out with RO water, under my RO spout in a bucket, until it runs clear. Sometimes it takes 10g to rinse the media, but it's worth it
When I put my reactor back online, I run the outlet into my socks for a couple mins. That's it
Dose the linked trace elements and all should be good
Of course, I'm just a newb, so take this for what it is ... my 5 cents