Its actually Dan as Fran is my wife.
If you create rock caves the snakes will enter and that is their seclusion, their security, their feeding post. Without it your snakes will die no matter the water quality. You also need other things in the cage so they can rest against when swimming around, plastic plants, sticks, other rocks, etc. They cannot swim like fish and eat like fish, then need something to grip their body against when striking a fish and want to pull it back into something to eat it and not have the struggling fish be attacked by other fish as the snake will assume its taking a chance itself latching on to a fish. As to your questions:
1. I don't think it's a good idae to planting water plants in the tank , the water plants will cause poor water condition , I offered them a nice filter system and acid water which mixed aquarium black water . I don't wanna planting any water plants i the water .... do you think this is possible ?
Well, I have consulted an experienced water person and he recommended the plants for two reasons. One being that the plants cause a natural purification of the water. Two, the plants are an indicator of good water quality. If you put two or three small plants in a small pot that holds them that should not be a mess and their decomposition will add to the acid water quality. Also providing proper lighting will help the plants grow and establish algae to also clean the tank.
2. I keep the ETS in a tank that size about 3 x 2 x 2 feets . Do you think this tank is too small for 1 pair 3 feets ETS ?
Well keeping them together is going to be a major problem as they will eventually stress and that will lead to blisters. Once blisters start, you can almost call it quits. Silvadine is a good treatment medication but most do not make it. You can keep tentacle snakes together without any problems as they are seemingly the only social snake, but elephant trunks are not.
3. I didn't placed any substrare in the tank for keep the better water condution . Besides hiding spot (cave) , I don't wanna place any item in the water . What do you think about this ?
You must keep several inches of crushed rock or gravel to establish a bacteria bed for better water quality. If you try to keep a sterile tank your going to run into more problems and more work. The decomposing waste material is what actually will help create a bacteria that will clear the water and keep it conditioned properly.
PS : Would left your e mail address or Microsoft MSN messenger account to me ? I emailed you so you can respond.