1st: Filtration is lacking! Please do tell us what filters you're actually using. You'd be lucky to even be hitting the minimum needed with a lighter stock.
2nd: Your temps are too high. You'd also be surprised at the amount the O2 that is depleted at those temps
Also, you seem to know everything already. Any advice given to you is met with a quick excuse as why "it can't be". From your responses. You should do a bit more research before you purchase your next fish.
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I have two 70 gallon tetra filters and a 100 gallon fluval canister filter. Do you mean filter media? And While I'm on rambling here I would like to ask everyone how I can improve my filtration to house a stingray. Don't worry, I won't plan on getting another ray until I know my filtration is good enough for a ray. I might not have space for a large sump though... I know how to meet with basic care needs with these fish but it finally appears to me I'm retarded when it comes to filtration XD