[vietmanx]

is 1 1/2 inch spa flex pvc safe for use in a freshwater tank with inline canister filters and a 1900 gph pump?

 

[three90s&125sump]

I would be more worried about the canister filters handling 1900GPH. The spa flex will be fine provided it is properly installed. I think you would want to go with 2" to reduce head presure. 1.5 is only good for 1800GPH IIRC. Bigger is better to a cirtain point.