Sorry to hear about your pandas. I went through losing them like that, too. Then I stopped losing any until I went away for 2 weeks on vacation. I did everything I could to prep the tank but the automatic feeders (Eheim brand) malfunctioned--not dispensing any food to my goldfish tank and overfeeding the cory/cichlid danio tank. The tank looked fine but had a bad algae bloom and the substrate was pretty skanky, and I should have changed it out right away, but being Eco-Complete, it was dark and I could not see that it was filthy. I mean, it looks like dirt. So how can you tell when dirt is dirty? I didn't see any poop. But when I was taking the substrate out, I found a lot of little bits of missing panda cories. And I continued to lose a few more until a week or so after the substrate change.
After lots of experimentation with different setups I find I lost pandas when...driftwood was very far deteriorated and I didn't realize it, plus it and all the plants trapped debris and gases and anerobic bacteria in the substrate. Everyone keeps planted and driftwood decorated tanks with no problems except me, apparently.
Or...I lost them when I let the sand stay in the tank over a month. So now I change out ALL of the sand and put in new sand once a month. I keep it to a depth of only 1/4 an inch now, since I am no longer trying to root plants in it. The fish are used to this and seem fine and even playful while I disrupt their living quarters like this. I don't even have to take them out, but past experience makes me sometimes dose the tank with Jungle Buddies Anti-Parasite if I notice them flashing shortly afterward. It doesn't always happen, especially now that I change once a month, but in the past it seems taking up the substrate released some parasites or some kind of no-see-ums that caused gill redness. Plain Praziquantel also works, too. I use Jungle Buddies because it has Prazi and other ingredients to tackle things I might not suspect.
I have tried almost all the sands. The one that seems to work best texture and size wise and seems to bring out the most snuffling in the sand activity has proven to be Estes Marine Sand. Yes, I know it says Marine sand but in fine print it says it is okay for freshwater tanks, too, and it most definitely is. I have also tried Tahitian moon sand in the black and the much finer white varieties. The black is perfectly fine but a bit more abrasive than the Estes sand so the snuffling activity seems to be less, and the white is very soft and fine but floats up into the filter intakes too easily.
I have also tried Eco Complete while trying to maintain a natural planted tank but it was while I was using Eco Complete that I experienced the most health problems in the panda cories and had an outbreak of fin rot (it was after the return from my vacation--after the tank had been cleaned, or so I thought, but before it occured to me to change out the EcoComplete).
I clean the tank every week and do water changes but that was not enough to keep such fine substrate clean. Even in my goldfish tank with small rounded gravel, despite regular gravel vacuuming, I had one goldfish come down with rot and septicemia. (He is a delicate fish, having been purchased in a weakened condition and nursed back to health). He pulled through very well after I changed out the substrate and reduced the depth to half an inch. I used to wonder if I overfeed but when I clean I don't see any leftover food and all of my fish are very thorough eaters. To me, it seems problems crop up when gravel substrate is deeper than an inch and sand substrate is up to a half an inch or slightly more.
I keep hot and cold water cories in together--sterba with pandas, for example. I keep the tank at 74F but have had no problems keeping temps at 78 and even 80 for weeks.
I don't have problems with initiating mini cycles every time I change out the substrate because I have such high capacity canister filters running on my tanks.
I also keep minimal decor now, and no hollow decorations that could trap debris or toxic gases. I gave up on having a planted tank and content myself with a small scattering of java ferns in each tank. I also gave up on silk plants just to improve water flow in the tank. There is now no ornamentation at all in the goldfish tank and only a "Tiki hut" and small bridge decor for the cories and one non-hollow cave decoration for my cichlid to live in.
My tanks no longer look natural, unfortunately, but the fish are healthy and happy and even gifted me with some fine albino cory eggs recently (I got to see them spawn and all that cool stuff!). Alas, the danios ate them all...the pigs.
I absolutely love pandas--they are ALWAYS active, outgoing and unflappable. I still have 5, out of the 11 I started with prior to the vacation troubles, and someday when I move everyone to a 75 or larger tank, I will try to boost them back up to 10. I also find my albino aeneus to have a similar playful outgoing and active disposition. Everyone else seems to like to cluster inside the tiki hut most of the time. Boring. But I think they made a sports bar in there.
Oh almost forgot--they are not necessarily that fragile. I accidently siphoned one up last month and he was stuck in the siphon hose, which I rested in an empty bucket, for about 15 minutes before I noticed him and rescued him. I put him back into the tank and he was busy looking for food a short time later. However I have noticed that 4 pandas are always together and one is now always by himself. I wonder if he came back with a tale of alien abduction and the other cories decided to give him some space.
PS...my signature is outdated so don't pay attention to the 3 pandas listed. I upped that number after I took a hiatus from the forum.
. Anyway if you can afford a QT tank, a would advise it, if not, just be very careful about the fish you buy. Research the fish you want first (I always do... look at my post on Hypancistrus debilittera - they screwed up