Epoxy would work. Certainly would be a permanent and easy solution.

Silicone rubber would probably hold the pipe easily. That is what
they use to seal the heaters on our spa and, of course, aquaria. The
biggest issue there would be tightness of fit. Tighter is better and
'play' can loosen the bond. Epoxy would not have that problem.

Will your new pond be outside? Above ground?

Would you want to have the drain of the spa draw water to your pump?

Jim