It is best to return the fish if you cannot provide a good setup as a group of gastromyzon will need a bigger tank than a 10g. You need enough room for each individual fish as they will be somewhat territorial to each other, even though they should be in groups. I find that different species prefer different parts of the tank. The g. ocellatus, I had, preferred the plants and rocks. The g. stellatus preferred the back of the tank and stayed only on the rocks, hiding most of the time. The g. ctenocephalus stayed on the plants and substrate. Also, you may want to be cautious about keeping the gastromyzon with mid or top level fish as I have seen these loaches be put into tanks and getting attacked or their skin eaten off. That's why I only kept mine with other gastromyzon and sewellia species. Have plenty of hiding places, especially smooth rocks, as they will chase each other.