New photos and/or vide of the fish please! With the lights on so we can see what we're looking at...So little update… they’ve stopped going crazy but now are just floating about and laid completely drained and lifeless. I’ve done 50% changes 3 times this week.
What are your test results for the water now, and what test kit are you using to test the water? Not all of them are that accurate.
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WHAT TO DO NOW?
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for a week. This should remove anything from the water that shouldn't be there.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
Try to get a video of the fish so we can see the behaviour. If you use a phone to film the fish, hold the phone horizontally so the footage fills the entire screen. You can upload videos to YouTube, then copy & paste the link here.
Why only 50% water changes 3 times per week? This is an emergency situation at this point, your loaches are dying. Why haven't you been doing the 75% daily W/C's and tank cleanings? It's the only to be sure the water isn't the main problem, would be diluting any contaminant that somehow contaminated the tank without your awareness, and would help the fishes immune systems fight off any other parasites or infections, since super clean fresh water helps their immune system handle any other issues, buying you time to find the underlying cause so treatment can be targeted.
Is it still a fear that doing large water changes will harm them somehow? Because it won't, and it certainly can't make them any worse than they are now.
Sadly no quarantine tank. I will be investing in one now, just never had an issue I can’t solve before
I did say that I can talk you through how to make a cheap temporary QT tank out of a food safe plastic storage tote. Can work brilliantly, especially in an emergency and don't have time to get and set up another tank, and with large fish that need bigger than a ten gal QT