I guess the only thing is that if it was leaking into the overflow with only 10g of water then it's probably worth addressing now. Ordinarily I wouldn't sweat a tiny leak into the overflow if the tank was full and the overflow empty, it just means the water would equalize into the overflow. But if you had a short standpipe or just a strainer on the bulkhead (eg. if it was a Herbie overflow) then potentially the water level in the tank is at risk in a power fail situation, so in that case I'd fix it. But I think if you're noticing water slip in with only 10g then it's probably going to be a pretty fast leak when the tank is completely full, so I think either way, better to say a few swear words now (Well OK that's only if you're me
), then err on the side of caution. It's easier to deal with this now (and not ever know if it's "really" needed) than to find out down the road when there's livestock in there that it IS a problem and you have to deal with it then.
Just my $0.02
Murphy's Law and all that