I'd hate to be the bearer of bad news, but a fifty gallon tank will not be enough for permanent keeping. Aquariums are terrible for keeping tortoises, for the simple fact that they hold too much heat. Before we get into that, however, we must first establish something. What sort of greek is this? Is it a Golden Greek (these are generally mostly gold or flesh colored; they are very sensitive to environmental conditions, most notably wetness and humidity, as they come from the dry deserts of Syria), is it a Turkish Greek (these are usually dark skinned, grayish brown shelled critters who are much hardier than the aforementioned Golden Greek), or is it a Lybian Greek (these animals have a blotchy pattern of black/brown on a yellow shell; they also tend to be rather larger than both goldens and turks, which are usually four to six inches at full size). Where did you get this animal? Is it eating? I recommend consulting the site Chelonia.org. They have a myriad of pictures, identifying the origin of the different types of greeks. Keep in mind there is still a great deal of dispute regarding the taxonomy the greek tortoise group (none of whom actually live in Greece; they're so named because Gray, the man who described them, thought their shell looked like a greek mosaic), and such makes their care requirements difficult to assess at first.
Okay, as for the animal's upkeep, you should build a tortoise table. I generally like to provide a decent sized environment, and my indoor tortoise tables measure two feet by four feet (the size of two fifty gallon aquaria!). Keep a spotlight with a fifty watt heat lamp over one side. Underfloor heating is necessary for Lybians and Goldens, but is unnatural for Turks, who are accustomed to nightly drops in temperature. This is, as you would imagine, very important, making determination of the animal's general origin of the utmost importance.
As for the feeding, ensure as great a variety as possible. Dandelions, clover, thistles, hibiscus, althea flowers, grape leaves, and other items listed in many caresheets available across the web (just search Greek tortoise care) should be offered (not all at the same time; simply revolve these items on an irregular schedule). Allow the animal some supervised outdoor time to browse for weeds on it's own, or build an outdoor enclosure to keep the animal outdoors during part of the year. I know no place in the continental U.S. where tortoises can't benefit from a few sunny days. Even in fifty degree weather, Greeks tend to do very well so long as it's sunny.
Anywho, I hope these suggestions have helped. If you have any further specific questions, don't be afraid to ask. If you have any pics of the animal in question, don't hesitate to post them either.
T.G.
