Would you say a 15 gallon tank is big enough to house a gecko?
I don't do gallon's but my tank is 2ft by 2ft think thats about 8-10 gallons? but thats big enough for one
What substrate should i use eg. sand .. ?
DO NOT use sand as substrate geckos can get impacted, your better off using cork, lino, kitchen roll E.t.c...
Is there need for drift wood etc ?
You could use drift wood its basicaly things for them to hide under or climb on or whatever
What is the best type of food to feed them?
Meal worms, locusts, Crickets.. A wax worm once or twice a week (Wax worms are like chocolate to them so dont give them to many they will get addicted to them, good for putting weight on geckos though)
Do they need supplements eg.. calcium supplement?
Cricket dust.. when you go to feed your gecko just dab the (cricket, worm, locust) into the powder and shake the excess off.. you can put liquid calcium into the water aswell
Basics:- Heat mat OR UV blub, you don't need both, people have bulbs mainly to see them.
A drinking bowl
Two hides: one for the cold part of the viv one for the hot part.
Rocks? Pebbles? Plants? anything that they can hide in or climb on basicaly.