Canreef Aquatics Bulletin Board - Help water problem

Honestly what I see here is an issue of too little substrate for things that would out-compete the gross crap in your water at this stage.

Your tank pics made me think back to my classes on resilience theory - one of the examples used in resilience theory writings is how benthic dominated lake systems (i.e., lake systems with clear water, low dissolved nutrients, and complex vascular plant life on the bottom) can absorb a certain amount of eutrophication without really changing in state - as nutrient inputs increase, they are taken up by an increase in the rate of growth of the plants on the bottom, keeping the total dissolved nutrient load relatively stable. However, once a threshold is passed and the nutrient inputs outpace the maximum uptake by the benthic plants, it opens the door for simple, pelagic, single celled micro-algaes to begin reproducing. When that happens, the entire lake ecosystem can rapidly shift, the water gets murky with floating algae, which blocks the light from hitting the bottom, which causes the vascular plants to die back, which leads to more available nutrients, etc. etc. in a positive feedback loop that results in a new stable state that is very difficult to reverse once achieved.

In your case, I see two smallish rocks relative to the total system, what appears to be a very fine/non-existent layer of sand, and at least 4 fish and a shrimp, which in this context I would consider to be a moderate/heavy bioload as the tank is currently set up.

You're going to have next to nothing in terms de-nitrification happening with the amount of rock and sand that you've got now, you don't have much calcareous material in there for phosphates to be binding to, and you don't have any sort of chemical/biomedia to be taking up nutrients, so it would appear that your only nutrient exports of any kind are via water changes and your stock skimmer. I've heard the RSM stock skimmer is notoriously inefficient, as seeing as it's not taking out that green stuff faster than it's reproducing, I think that's evident.

With no exports, even a very conservative feeding regime can produce significant dissolved nutrients in a small closed box. With next to no substrate for macro-algaes to be growing on, limited substrate for any 'natural' nutrient reduction via chemical or bacterial processes, and mechanical filtration processes (water changes/skimmer) that removes that green stuff slower than it reproduces, that leaves the door wide open for simple, single celled pelagic algae (aka phytoplankton) to go nuts. Of course your nutrients are still reading zero, because all those nitrogen and phosphorous atoms are bound up inside the structures and DNA of the micro algae. I bet if you took some of that green water, froze it, then let it sit out for a few days and then tested it again, you'd get N and P readings way outside the acceptable reef range.

This reads to me like that case I described from the resilience literature, only your problem is that there's just not enough substrate for the 'benthic' state to ever have happened. You've basically achieved by accident what people who try to culture phytoplankton bang their heads against walls trying to make happen.

If I were you, I'd either -

a) wait. This might eventually sort itself out, but that's asking a lot of two pieces of rock and I'm not sure they'll ever get there from a biological point of view. I actually wouldn't do this but it is an option so I thought I'd mention it.

b) get a better skimmer. A skimmer that's working efficiently should have no problem removing 100% of the free floating single celled guys, but that will just leave more nutrients for the green stuff to grow on your glass

c) at least triple the amount of rock you have, add a thicker layer of sand, or both. Doesn't even have to be 'live' rock, but if you do it with dead rock it will be a while before it starts to help in any significant way

d) both b and c. If you've got no ability to add nutrient processing infrastructure in the form of a GFO reactor or a biopellet reactor, you're basically going to have rely on the 'berlin method' for this tank. The Berlin method requires a good skimmer, lots of high quality rock, and a bio-load appropriate for the kind of nutrient regime you're trying to achieve.

Ultimately if you don't set up a system to account for nutrients, problem algae (be that micro/freefloating or macro) will account for it for you.

And this has been another novel from your friend and mine, member asylumdown.