Did you quarantine your discus fish? And you say you added the corydora two days ago?
Matt
Honestly, 6 discus in a 60 gallon with only 25% water change each week, you really need to be changing that water daily or at least once every other day at about 75%. With the other stuff you have in there such as the Driftwood, rocks, substrate, and other fish, then it's really less than 60 gallons of actual water. By the time you change the water then I'm thinking the water parameters are very different in the tank then what's going into the tank and that can wreak havoc on them. Just try 50% WC for 4 days straight and see if they are lively and buzzing around by day 4. Keep light on but that corner seems much darker than the rest of the tank -- maybe redistribute shading so it's near even or sporadic throughout tank?
-Elliot
+1 on give them time. You've already skipped quarantine, so it's a matter of keeping it clean, as in water changes, without doing so much at once that it stresses them. I like the 50% every couple days idea. Also, I think your rock and sand setup will create pockets for debris to accumulate. I recommend "disturbing" it all regularly, just enough to siphon your sand and move around any trapped waste. Those guys are still in the juvie stage, so keep monitoring the conditions and do more water changes.
Before reading this I had recognised that the lighting and distribution of cover was a bit sparse, so I’ve since changed that and the lighting, and increased the WCs and it’s made a big difference. Not sure if it’s the water so much because as soon as I had redone the tank they were out and about with the light on- something they didn’t do before, but I’m doing 30% every other day. Much more confident now.
Nice! Just keep figuring out ways to make your WCs easier on you and they'll stay happy and you'll stay committed. I'm definitely a hypocrite when I say that since the past year I've slacked a little but to me that's the #1 thing to keep them lively, happy, and healthy. Right now, mine are flicking me off.
-Elliot
My tank is the same size, had my discus lay eggs this week and are healthy. I do a total gravel vac , toothbrush hardware m hardscape, clean glass , and do a 75% water change weekly. Ammonia 0ppm nitrite 0 ppm nitrate 10-15 ppm 0 issues. I have 4 discus 2 juvi , two adult and about 26 tetras. Im the first to admit compared to these guys I Screenshot_20200410-101228_Gallery.jpgScreenshot_20200410-101228_Gallery.jpgam a noob. I've had tanks for over 16 years and discus one time before for 8 years, but still noob. That said... i don't think 50% water changes 2 and 3 times a week is necessary. I don't do that and my water testing does not show a need for it. Nor does the fish behavior ( attempting to breed after all). All depends of bio filtration etc.