Short version: I got a used fish tank which is 'kin huge, including an overflow into sump. There is a hole drilled in the base of the tank. A tank connector I believe they’re called was fitted as supplied, but when I leak tested it, it leaked. Seller also gave me a new one. So after much pain, got the old one out, in with the new. Leaked even faster. I believe it is coming from between connector and tank, not between connector and pipe.
Spoke to guy at work, he said if the tank connector moves at all, it’s too small. Might get away with it if I perfectly fit it centrally. The one I had in pic above was apparently a standard plumbing size. After much searching, I found a similar part for aquarium use that sounded closer. Ordered it (£1.49) and arrived today. It doesn’t fit. Bit too big. I don’t think there’s an exact size for the hole in my tank.
Not sure what to do next. It has been suggested I bin the rubber seal and instead use a ton of silicone sealant.