1.The type of test kit you are using aquarium Pharm master
2. before and after water change.
PH 8.0 (note tap water is 7. (issue)
Ammonia 0
Nitrite old chems did not test
Nitrate 20ppm after water change yesterday (issue)
3.Water temp 80
4.Type and depth of substrate (1/8 inch silica quartz)
5.Tankmates and size of the tankmates (P-bass and hoplias curupira)
6.What and how often the ray eats (just got him, 1st day ate salmon, silversides. Not super active hunting for food.)
7.How long you have been caring for the ray (New)
8.How long your tank has been set up and your water change schedule (how much and how often) set up for 3 weeks, carried filter media from old tank for bio) Water changes 2/week 15%or 60gal)
9.method of aeration of water 150 gallon wetdry
10. Filtration type 150 wetdry
11. Picture of tank having problem if you can. (Helps with knowing your set up)
12. tank size (gallons or leters / and footprint) 600gal 8' by 30"
So it looks like the nitrate is high (water changes more freq) and substrate is affecting the PH because tap water is 7.4.