Personally, I prefer not to use gravel for my turtle. Heard too many cases where turtles have consumed the gravel and have become compacted. I generally use large rocks or river rocks for that reason. Still gives the turtle substrate to push around but no risk.
Here are some pictures of my turtles current tank. At the moment he's in a 4ft x 18inch x 15 inch tank, which has about 175 actual litres of water in it (as full as it can go without risk of turt getting out. The tank also has a 1500LPH canister filter which does its job perfectly, keeps the ammonia and nitrites down and keeps the tank crystal clear. Turt (thats his name.. don't laugh, name wasn't my fault. blame my father lol) is a murray shortneck, is about 20cm in carapace length and is 1.5 years old.
Overall view of the tank. before i get comments, im aware the heater is useless for this size tank. i had two in at the time, because this was soon after i had set the tank up, when i bought it. hadnt yet bought a larger heater.
Above view of right side of tank. shows lilly leaves and piece of free-floating driftwood. turt loves to rest on this as it floats around.
Part of the left side of the tank. you can see the crappy floating zoo med turtle dock. I hate these things. The suction cups arnt great, the bottom metal plate drops off, it has a pasty residue on the bottom when in the water, when you have a larger turtle the things sink.. i just hate them.
Top view of the left side of the tank, showing the crappy dock again and the lilly leaves, with some plants wedged behind the dock.
Some rocks and plants in the tank
I scrapped the dodgy zoomed dock and made my own. used acrylic, siliconed on white rocks/pebbles and siliconed it into teh tank. no suction cup problems, very secure, dosnt sink, dosnt fall apart, and most of all, turt likes it, so im happy.
And, turt himself. My little (or.. not so little anymore) spoilt boy
Personally I think the more water a turtle has, the better. I mean in the wild they live in massive dams, and love to swim. Turt has actually outgrown his current tank, and I had planned on getting him a new one in my last university lecture recess. However that didnt happen. Now I actually have a 4ft x 2ft x 2ft tank (capacity of 453L) sitting at home waiting for me to go home for the june/july holidays (Im in townsville uni studying BVSc, home is in brissie) The diagram below shows my basic plan for the basking area of the tank.
Im doing an above tank basking area to give turt all the swimming space possible i.e. the water depth will be to the rim of the tank itself. also, nice big basking area which i will be siliconing river rocks onto for grip and a kind of semi natural look (better than carpet or anything in my opinion. dont like that look) to add the acrylic extensions (will get clear stuff) i'll probably use hinges on the front and right side, with clips at the top which can be fastened to keep the walls up. however if its hinged it makes it a lot easier to access the tank, as i can just undo the clips, drop the walls down and clean the tank or whatever.
Large smooth black river rocks will be used as the substrate, with other large rocks (such as the ones in the picture above, but smooth. those large rocks have been thrown out and There are now large, smooth sandstone rocks in there) and there will be plenty of live plants for turt to eat (he loves his vegetation) Will also have driftwood for him to rest on and explore around.
Also... arnt longnecks large turtles when theyre adults? a 3ft tank isnt very big for a large turtle to live in...