Combine vinegar and sugar in a small saucepan; cook over medium heat 5 minutes or until sugar dissolves, stirring occasionally. Remove from heat; let cool.
Combine cooled vinegar mixture, cucumber, and next 5 ingredients; toss well. Let stand 30 minutes.
Heat a large nonstick skillet over medium- high heat. Sprinkle fillets with salt; coat fillets with cooking spray. Cook fillets in batches 4 minutes on each side or until fish flakes easily when tested with a fork. Spoon cucumber relish over catfish; serve immediately.