Help! One fish died, other isn't looking good

Hey I wasnt expecting this but this week my fish Jekyll is looking bad. My dad takes care of them since I don't live at home anymore. And I'm concerned that he isn't willing to do any treatments to help. Ill take any advice.

I'm trying to get water parameters but dad is reluctant 😕 he is kind of convinced that the pothos plants keep the water perfect so we have this argument on water quality a lot. It's never been horrible water though and I believe he did a water change last week.

Signs of disease: redness in fins and lethargy Signs of depression post fish friend death: lethargy

No swim bladder problems. He's just sitting on the ground and sad looking