my discus are ill and im getting ill as well!

[XIHOHORORORO]

Hello guys, I got a problem and IDISCUSSSS.jpg don't know what's happening anymore.
Started noticing some discus getting dark, they stopped eating and pooping a short white poop. it was a really weird poop and I have never seen it before. it doesn't stick to the anus like normal poop. The moment it comes out, it immediately falls down and seems like it falls faster and quicker than normal poop. its really short, at the beginning I thought it was sand stick to their body but after observing for a while I noticed it. It happened 1 week ago after a big water change

notice how they look vertically and cant stay in normal position. they try to keep normal but they start heading vertically. anyone knows it? whoever can help me I can literally give a generous tip to him/her .

PLs Im so desesperate.

[XIHOHORORORO]

I use prime and I usually put some seachem stability and stress guard after big changes, prime every water change.

[Iminit]

How often have you been changing water? And how much? Is that algae growing on the sand? If so when removed does it have a very strong smell? How are your water tests?