Papa Splax

Hey I posted my problem in New to hobby named (fish keep dyin please help) Long story short everythings perfect fish die mysterously bwithin 10hrs no symtoms... Again other post in New to hobby goes way more in depth with setup parameters the whole 9 yards Please help

I have as follows 55g w. Overflow X2 600gph powerheads X2 heaters (74) T5 36" lights 30G 3 chamber refugium (1) Bio balls (2) 6in sand bed w. LR & Sea grass (3) Gravel, LR, return pump Paramenters 0 ammonia 380 calcium 0 nitrates 1.023 saltinty 8.0 ph Problem... NO matter where I get a fish it dies (w. Proper acclimation) I was told maybe Oxygen deprevastion so I thru in a air pump with a rock at the end to make buBbles Smaller and let it run for 3 days and fish stiLl dies within 10 hrs... While looking very healthy only symptom is heavy breeteing... Also have like 10 baby anenomes thriving, horseshoe crab, and snail... And some LR worm... BUT all fish die (except one tester mollies which is doing ok wierd enough)