Rescue fish from strangers are russian roulette. Sometimes you have to be hard hearted.
Why are they alone?
It could be they were bought by someone who didn't do their homework first. But it could also be that someone bought ten, which carried an infection that killed nine and may be dormant and held in check by the survivor (but unencountered by your fish). It isn't uncommon for a fishkeeper to have a healthy tank of wild caught fish, add one healthy farm raised fish (which will have been exposed to numerous crowding related diseases and survived them) and have everything die but the farm fish. It's a first contact scenario.
If you are going to undertake rescues, you have to setup for it. That means a quarantine tank, cycled, at the right temperature, and set up as a safe and healthy environment for new arrivals. I see the logic in the plastic tub idea, but you need a tank. You have to be able to monitor the health of the new arrival for around 6 weeks. It can't be an environment that brings on illness. If the quarantine tank causes the death of the fish, then you've done it wrong.
It's an expense, and space, but if you're going to do it, do it right. Otherwise, I'd advise hardening your heart and taking care of the group you have.
And please - there is no such thing as a rescue from a pet shop. People buy fish from stores they shouldn't support, and then claim it as a virtue. The stores are happy and have no reason to improve, and the buyer gets a fish that is a potential disease bomb in their tanks.
Our hobby is based on living animals being sold as commodities, and we have to navigate that with a priority on providing proper care for the creatures we have now. QT for new arrivals is part of that. Without it, I wouldn't go there.
