Hey guys, for starters thanks for all the replies.
I did think of tannin at first, as I did add a big chuck of wood, but as I said, I've been having the problem before it was added, with only 2 pieces of wood that had been there for about a year in another tank with no issue.
I do have a small powerhead, but I am trying to keep the flow to the minimum, until my pleuros get there anyway.
According to what I've just read on Eheim website, I believe mine cycles twice my entire tank per hour... I know it's the bare minimum, but I was pretty sure it would be ok... In hindsight, I may very well be wrong...
As for feeding, it has happenned through my 2 eras. One with african cichlids, that had their own 4 different foods, from 4 different brands, and now with what is listed above, eating 3 different kind of food, with occasional frozen bloodworms (I find frozen krill to be rather messy).
And as for lighting, 3 led tubes, one full white, one white+blue, one full blue. Blue to start and finish the day, as it's very dim, mixed one half an hour later, full white an hour later (and earlier). I should probably get the exact whatever the measurement is (lumen is it?) for them, but I assume they are made for this size tank, being sold by the guy who made my custom tank.
All these could be causes, though I don't believe much in the food section, and I can try to fix all of them, one week after the other, besides the filtration, which I find to be the most plausible cause, but will require a bit of time, money being a little tight for a couple of months...
What strikes me the most is that tannin filled water tends to "appear" even more "pure" than water right out of the sink, and it is not the same in this case, it does look a bit "dirty". If it had been tanin, i'd actually have liked that, as I find it amazing looking.
As for the last thing I noticed, I don't know what purigen is, but if it can "fix" my issue, I am assuming it would still not be as good as actually fixing the root cause, which I would rather do... If it's just because of tanin though, the root cause isn't that bad, and harmless for the fishes, in which case I would gladly consider it!
I hope it helps, and again, thanks for all the replies!