I've had ottos do fine in a 10g before. You just have to be super good about water changes and such. They need to be in a group, and they are super picky about water quality. Also, you will need to feed them. They will eat any algae in the tank, but also zucchini and leafy green veggies offered daily are a must--especially if the tank doesn't have a lot of algae. These items must not stay in the tank too long or they will foul the water. But, they need to be in long enough for the ottos to eat. If you do get ottos, try to get the ones that have been at the pet store the longest, as the ones that have just arrived will be the most stressed out, and will get more stressed moving again to your tank.