MM, my reef ready mega flow 75g only held 61g after LR and sand. If your tank is the same with an internal overflow (Aqueon) it most likely only holds approx. 75ish gallons, or even less if you're LR heavy.
And when you say you've drained it down to 5" keep in mind that that last 5" empty might hold 18ish gallons, but it's much less with sand and rock.
Honestly it sounds like your drum is more than 50g. My blue barrel is 55g near full but will hold almost 60g.
Stamping on it reads 200 litres, but I've filled it a few inches from the top and it's 55g at that point.
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