Hmmm, I've racked my brain over this topic since the thread got posted! There haven't been any I really regret, yet.
Perhaps one. I bought a single pygmy cory I spotted while at the LFS for food and things, because I knew I wanted some later down the road, and he was the last from a stock my LFS had in. Someone had bought a load of the batch, leaving a little guy all alone. I felt bad for him, and he was so cute, I bought him, thinking it wouldn't be long until I could get some more, and that maybe he'd hang with my otos in the meantime. I'd seen videos of otos and pygmy cories hanging out together, following one another, it was adorable. Asked my LFS guy to order some more as soon as he could.
But with Brexit and Covid, store was having supply problems. I kept asking, they kept telling me they couldn't get any at the moment, but they'd keep trying, that others wanted some too. Larry the Loner as I dubbed him zipped about the tank, seemed okay, but didn't really bother with the otos much. After a while of this, called the only two other fish stores anywhere near me, chains, and they didn't have any. I ended up raising the GH slightly, so the tank water would be on the high end of acceptable for otos and pygmy cories, and the low end for guppies, and added some male guppies, hoping some mid -water swimmers would keep him company. That seemed to help for a while, he did swim with them sometimes, but I knew it wasn't the same as having other pygmies.
A couple of months pass, and I'm starting to worry. I look into online stores. Find out that shipping would understandably cost as much as the school of fish did, and I just couldn't afford that all at once.
Then Larry started to seem depressed. Stopped pottering about the tank and sitting on leaves, and sat on the filter intake, rarely moving from that spot. He wasn't sick, tank water was perfect, he was lonely, I'm sure of it. Luckily only a few days after that, the store let me know they'd been able to order some in, and I placed a deposit straight away! Night before I go to pick them up, they let me know that someone had accidentally sold a load, they had two left. Really apologetic, I understand, things happen. I go to pick them up, they're even more apologetic, only one left. Filter accident. But they've ordered more in, due in a week. One is better than nothing, he looks healthy, I take him home. He's half the size of Larry, only a baby! Larry does seem to perk up a little. The next week, I pick up six more, also babies the size of the second one, adorable, and Larry is definitely much happier being one of eight! They all sit about on the sand beach together, and mill about in groups of three or four when they fancy a swim, Larry among them. And now, they do hang out with the otos! Often find all of them sitting on the sand together. Will be getting another batch of pygmies soon to bump them up to 12 or so.
But from getting Larry, to finally having a decent sized school, took four months. In hindsight, I wish I'd left him in the store. There's a good chance someone who already had a load of pygmies would have picked him up, I know I'm not the only one who feels sorry for singletons left in tanks. Lesson learned there!
