Will fishes survive if put with tortoises?

Are these actual tortoises, or are they turtles? Tortoises are desert creatures and not aquatic at all. They should not be kept in or on water, they come from very dry environments and can become sick if kept in wet or moist conditions. Their shells can not handle being wet for extended periods. They can end up with fungus or shell rot. Are you talking about putting the guppies in their water dish maybe? Actual guppies are tropical, and do best in warm temperature controlled water. So this would not be beneficial to the guppies. Guppies in a water dish may be tempting for the tortoise or turtles to eat. They certainly wouldn't look at the guppies as "playmates," as they are not human. Tortoises are generally not social creatures in the wild. If you have two of them, and you give them the attention they need, they will not be "lonely." Tortoises do not "play," they wander from place to place in search of food. Now if your tortoises are actual TURTLES, it could be a different story, but guppies in their water would still equal potential snack.

I'm not that familiar with tortoises, but I would say it's probably not a good idea. Does the tortoise go to the bathroom in the water? I just see the water not being very clean for a fish to survive, especially shallow water. Guppies will need more water then that to survive.

Guppies, or any other fish for that matter, need a full aquarium of heated, filtered, and treated water. It would not be a good idea to put them in the 3 inches of water in your turtle tank. The water conditions wouldn't be good for them, and they wouldn't have enough room to swim around.