Used WaterBox 90.3 clean

[steak.mama]

We’ve upgraded! We went from a 15 gallon and just bought a used WaterBox 90.3 and it has as little bit of build up on the glass, the sump box has a lot more!
I wanted to use citric acid, but because it’s so big and heavy, I don’t know how I can rinse it properly so it won’t affect the tank once I add water? Is there a best way to do this? Thanks for your help!

 

[blaxsun]

I know what I would do, which would be place a blanket on the stand, tip it on it's back and work on scraping the algae out at a 90° angle (instead of 180°), which would allow me to do this with a minimal amount of water, etc.