The subject of UVB lighting for reptiles can be confusing at times. A company in the UK tests the quality of a wide range of reptile bulbs available throughout the world. The following are excerpts from emails I've sent to Francis Baines on the subject. You can check out their web site on http://www.uvguide.co.uk
With a male, you can get away with a 4ft tank (120cm) ....but only if the water is very deep and tank is not narrow.
A 5ft is ideal and a 6ft is excellent. Even a baby needs at LEAST a 3 ft (90cm) tank and that should be changed to a 4ft (120cm) by the time the turtle is a year old. To save money its best to get a larger tank to start with! Photo is of a 6 ft tank. The bigger the tank the better.
A large external filter is needed as turtles are very messy. Partial water changes are needed every week to stop turtle living in their own filth. Shingle is a huge killer of turtles... so plain tank bottom is best. Driftwood is good for them to scratch, chew, rub against to help remove scutes etc. Rocks can lead to ulcers, crush injuries and possible drowning.
Need 2 lights. A UVB reptile light ... if it doesnt say reptile, its no good. You also need an ordinary incandescent bulb or a reptile heatlamp for a 'heat' source to encourage turtle out of the water.
UVB bulb needs replacing 6 to 12 monthly.
This tank is suitable for ONE male turtle, and although only 120cm x 50cm x 50 cm, you can get away with it for the life of the turtle as the water level is high. The basking end of tank is 60cm high with a 'topper'. Basking ramp is positioned 25cm from the top of the tank to allow for lighting to be placed directly on top. Basking area is 35cm from base of tank. Bigger is better and if you can, please make your tank larger then this one.