The Wait float valves are great, I've used them for topup for close to 15 years now on other tanks and there hasn't been any failure with them other than blocked lines or human error. KISS principle is better sometmes!
But on this tank, the topup enters the tank by being injected into the skimmer air intake. This has the added value that I don't ever need to clean the venturi. Two years in on this skimmer and every time I inspect the venturi it's sparkly clean. Every other skimmer I've had to do vinegar baths on the venturi's every couple of months so I know this is working.
I have a 12vdc solenoid valve coming from an ebay vendor but I have doubts it will get to me in time before I have to go away for a few days this weekend. Thus the need for something in the interim.
Tough call though. If it's $40 for a relay, I'd be further ahead to just find a LFS in town with the Tunze pumps in stock and pay $25 instead.
I was sort of hoping a relay would be more like $5.
Oh well. Shows you what I know..