Okay -- here's my two cents on all of the above based on setting up a similar tank (actually two different times with two different sizes. You can see pics of some of the tings I tried in my build thread.
First egg crate won't even stop a medium to med-large Nem. It takes them 30 seconds to get through. The thing to remember is that Nems are 95% water AND they can expel all the water to get tiny. After finding out that egg crate wasn't the slightest hindrance to getting behind the power head wall I went with 1'4" plastic netting. The same stuff used to make a DIY top for the tank. That added perhaps 30 more seconds for a Nem top squeeze through. Next, I overlapped sheets of the 1/4 inch so that the holes were much smaller than 1/8 of an inch. This worked for large nems, but small ones still got through, but the biggest problem with it was there was almost no flow through it and it got covered in algae and grime quickly.
Next step was the usual baskets. I have some very expensive Nem containers with small drilled holes in them and small Nems still get through them plus they get dirty.
So -- here's where I ended up. For most of the tank with med and large Nems
@grizfyrfyter power head covers work fantastic. For very small Nems I used small pond baskets till they grew a bit and could be moved.
This worked for me in bigger tanks, but my advice to you in a 20 gallon would be no power heads at all. Use a stronger return pump and it will be plenty of flow for the nems.
now, as to the marine pure balls I used those in the first Nem tank and they sucked. Not because of the aluminum issue. I, and many people, use them successfully in SPS and other tanks. I researched the issue pretty carefully and have no worries at all about Aluminum from them. Long and short of it is 1) the leaching is a short term issue, 2) there are actually two types of Aluminum and the type that may or may not come from MarinePure has no known toxicity to reef inhabitants.
The issue I had with them is Nems hate them. I watched the BRS video where they set-up an early Nem propagation tank using them and went all in. You'll notice they've never mentioned it again because it's not a great fit with Nems. The damn balls move around too much even if wedged in pretty well and Nems can't stand lack of stability. If they are large enough t attach to the glass below the balls they will, but they still end up moving around constantly. Moving Nems aren't happy Nems. You're better off using a 4" brick sitting on the bottom for nitrate control.
Nem tanks take a lot of work. People with a few Nems in a larger system don't realize how much slime and excrement they put out on a near nightly level. Your going to have to siphon stuff out at least every two days. You should be running an oversize skimmer and I highly recommend running a reactor with carbon at all times.
If I can help in any other way let me know.
thanks for the suggestion!
