sorry, i do not disagree with eveything you say, just in this case and a couple of others where opinion is involve(i also have not responded in contest to many of your other posts, for reason, i may even in agrrement, if i have not yet, it willl happen sometime, i am not seeking conflict, jsut offering my opinion), but i think i started here, by suggesting that a drilled in overflow in an empty tank is better, IMO, and then i just explained in more detail as to why, and if the power is lost, they DO loose their prime, you cannot flood proof 100% a hang on over flow, they are great if you cannot get a built in, but not as reliabe, you can drill a hole in th out take under teh surface of the water, which could act as a safety net against flooding the sump, and you can get a pump switch for the return pump, and all are certain aids, i must admit if you don't have another =option yes they are good, but if you have the option of built in they are more trustworthy and less sighlty, you can buy them as cheap as 25 or 30 dollars yes, you can also spend well over a hundred, drilling is normally 25 ot 75 around here, i was jsut stating my opninion on the overflows, one which i do have merit for my opinions, and they are the reason i do not have a hang on overflow and wish i had one built in to my 75 gallon tank, like i said, the hang ons do have their place, but i f i had an empty tank i woudl have it built in, that is all, and i do agree with you that:
NO they are not guarenteed to overflow, i may be a little too paranoid but they are not as trust worthy that is all , SORRY IF YOU FEEL THAT WAY