Here is one concern I have with drilling through the wall cavity (through the sheetrock) through the foam block then into the concrete;
To support properly you will want the item to be supported (TV or fish stand) bolted tight to the concrete. The farther the item is from the concrete the more leverage that can be applied and the more likely that the bolt will pull out. Think of those picture hangers that only use a few small nails through the metal plate that you hang the picture from, because the nails are at a downward angle and the plate is pressed to the wall with gravity they can hold a lot of weight, remove the plate and the nail just goes through the sheetrock and your picture falls to the ground.
So what if you took a piece of plywood and screwed it to two of the studs so it spanned the weight on two of the metal studs, then bolted this to the concrete. Essentially you will have the bolts in the concrete to support the item from pulling away from the wall and the metal studs can support the vertical shear force, this should give you a solid base to mount anything to. If you have a solid panel in the stand you may be able to use the stand as the piece of plywood I mentioned.
How large of a tank? Is it a deep or tall tank? My 72 gal was tall and not as stable but the 400 Gal was wide and deep, it was tall but there was a lot of mass on the base to keep it on the stand. One issue is keeping the stand in place, the other is keeping the tank on the stand. If acrylic you can more easily mount braces to the wall, but if glass you want to make sure it does not give a pressure point. Imagine if you had an earthquake and the tank got rocking, the small support would act like a crowbar, crack the tank and kill everything. One solution is if your hood comes over the front lip of the tank, you can support the hood to the wall, this will also help keep the tank in place and reduce the height of the swinging mass.
I know some of those ideas are overkill, but a big tank is a huge expense, they are like family and even 60 gallons of water will destroy several rooms… I know, we had a house fire (old electrical in the attic we did not touch) the firemen only used 60 gal of water and it took out two stories, kitchen, flooring, bathrooms, and so much more. Alaska is humid like Oregon and it will take too long to dry before the mold sets in so you will need to remove everything to get the water out. Trust me on that one, it is not fun!
I hope that helps a bit, good luck!
Ex Realtor, house flipper, remodelor and now building a vintage truck, making friends and fixing my house.
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