I have a 90g AGA tank with the megaflow overflow that they currently put in the AGA and Oceanic tanks. I am also planning to use a DSB. But the lower intake on the overflow is in the 4-5" range so it would be covered by the DSB.

Will the other two sets of holes still allow adequate flow (may approach 600gph)?

Are there any other issues that can happen with this overflow?

I have seen some of the mods where it is cut out for more flow, but I would prefer to stay away from that option.

As a side question, is it common to cover up the overflow to keep any jumpers or crawlers out? I thought this seemed liek a good idea but think I only saw it mentioned once.

dont know the unit your talking about, kinda sucks not being subjected to the same gear, but thems the breaks! why dont u want to modify>? it might be your only option cause u cant restrict flow, esspecially cause id imagine your have to modify it anyways with a dsb so u dont suck sand in! but theres the problem i cant understand, what no baffle in this unit>?
as for the sealed overflows, ive not seen them completely sealed, i have seen a shelf, or removable shelf above them for jumpers & snails, but more to the point coverings are used to sheld agains light getting in the baffle for limiting algae growth, 2mm gap clearance is the least i would recommend just in case the overflow teeth become clogged & it can overflow over the spillway!
im flying blind without a visual que or knowledge of the product in question anyways, might have answered your question in part anyways

Thanks V! Good point on some clearance incase of clog and darkening to keep stuff from growing. Here's the product link:
http://www.all-glass.com/products/aquariums/megaflow.shtml
http://www.all-glass.com/products/aquariums/megaflow_demo.shtml

I think I will rig some type of cover, Or the eggcrate might sit close enough to just attach a light blocking panel to it above the overflow. I don't mind doing the mod if it has to be done, but would prefer not too. I also didn't want to get it set up and then discover that I need to cut it up. But now that I think about it, the plastic shavings would float, so even setup it shouldnt be a big deal to cleanup.

Is there anyone out there actually using a megaflow with a DSB that covers the lowest intake?

Well I just did a quick calculation on the volume through the top holes only...

Assuming 24 holes each are at least 1/16" x 1/2" comes out to .75 total area. This is nearly the size of the 1" return (.785) and does not count the middle holes. So I figure it will probably be ok.

I will check at home to see for sure, but I believe the holes are actually larger than my assumptions.

man u guys get some cool add-ons in america. so u have water in this tank already?
but how does that product stand up in market place? i see the good & bad sides to drawing water in 3 places. and a bead of silicon right down the bottom will seal those holes if u needed it covered! its only going to be covered by sand anyways yeah.

That's the standard overflow used in All-Glass Aquariums (AGA) and Oceanic. (Both are owned by the same parent, but Oceanic are slightly more premium or were at one time at least.) Those probably the most common standard glass tanks with overflows sold here. But I haven't heard much pro or con on them.

I do not have my tank filled yet. I only bought it because I got a really good deal. I will not be filling it for a few months yet. The wait really is killing me, but I feel more prepared every week. I keep learning things that make me glad I didn't start already and do something that might not be best for the goals I have for my tank.

Checked the holes... there are 23 and they are just shy of 1/8" and actually over 7/8" long... so that comes to 2.5... Even if that is slightly high the top holes provide more than twice the area of the return line. So I guess I am pretty confident in it. If anything I coudl just drill a few extra holes in the sides or even anywhere lower really.

I understand the point of putting the overflow at different levels in order to reduce dead spots in the tank, but personally I don't know that this is the best way to do it.
The principle of good skimming relies on taking off the surface water layer, where the highest concentration of proteins will collect. If you're pulling water from mid- and deep layers of the tank, then you'll be reducing the efficiency of getting the uppermost surface cycled through the skimmer/sump.

I don't know if it will work with your tank, but I'd cover up the lower holes if the flow can be handled by the upper ones. You can reduce your turnover through the sump to help. Then I would rather place your returns at different levels in the tank to take care of the "dead spots". Would this be possible with your tank?

The lower holes will be covered by the DSB so not much flowing there. the middle section only has a few holes, so not a lot of flow in there. So most of the overflow will come from the surface.

I plan to use powerheads to take care of the in-tank circulation. I think I would like to use two Tunze stream (1600gph) alternating on a wavemaker and use a smaller pump and the directed return to make sure there are no dead spots in the back.

sounds like you've got it made.

I also plan on using Tunze streams for my new set-up. I can't wait until my hubby gets back from Germany next week with them so that I can try them out in my old set-up. I'll also have them hooked up to tide simulation....