90 gallon deep blue with built in overflow help

[jeanlee000]

I am upgrading to a 90 gallon tank from my 40 breeder. I purchased a 90 gallon deep blue with built in overflow. I am a little confused about what to put into the overflow. When I assembled the plumbing kit, the drainage opening that leads to the sump is located at the top of the overflow. What would I fill the overflow compartment with to prevent gunk from accumulating at the bottom.

 

[edd]

you have to syphon it out. i get a small amount of sand and small snails. i do it when it needs to cleaned, about 6 months or so. i think some people use a shop vac.