Oh man, always good to see someone else crazy enough to dunk their most valuable electronics in a fishtank! I'm also messing with Novec in an immersion cooling project now - I'm using Novec 649 to cool bitcoin miners, as its boiling point of 49°C is right at a normal operating temperature for a lot of ASICs. So far the results are great, although I would definitely look at using a more specialized enclosure - unlike mineral oil you won't get any benefit from having a lot of idle fluid/thermal mass, and it will run up the cost of your project very quickly. The tank I'm using is custom designed to hold as many ASIC boards as I can fit into it with just enough space between them to allow for bubbles to rise easily, and the recondensed fluid is piped to the bottom of the tank and reintroduced through holes between the boards. This minimizes the amount of fluid needed and keeps it constantly circulating without a pump.
In production:
In need of TLC:
In need of TLC: