In planning a few tweaks to my filter, I needed to put some barriers. For sump baffles as well as as a box for a static k1 media bed. Am using mainly cutouts from a 1/4" 4x8' sheet.

I need to use angle bars fastened by stainless tap screws to the concrete walls. 1/16" strips would line the gap between the bar and the concrete wall, as well as the gap between the PP board and the bar; where it acts as a gasket. Since I'm not versed with plastic welding, I plan on using sealants to join sides for the k1 filter box, with stainless corner braces for dimensional stability.

I'm inclined to use 3/4" aluminum angle bars because they're cheaper, and to a lesser extent, because they're more easily drilled than stainless steel. But I'm not sure about exposing the pond water continually to aluminum. Are there any known risks with aluminum? Do I specify a certain grade? Thanks.