My main questions are :
1. I want to have a very well planted tank. I would like suggestions on what plants to get, where to put them in the tank.
2. what kind of decoration should I get, rocks, bogwood, etc and also where they should be placed.
3. I would like a very dark substrate that is not too fine but not too course either. What have people used and what is preferable.
4. I would like to have the CO2 going into the external filter to mix with the water so there won't have to be any pipes and other devices in the tank. Has anyone done this and if so what is the safest way of doing it (I presume splicing into the inlet pipe and feeding the CO2 pipe into that and sealing it up again ?)
If people have any pictures of setups of corner tanks to give me ideas that would be great.
Ill try and see what I can answer 1 by 1... mind you, I am not pro just giving you my advice from little experience I have in planted tanks.
1. Well, plants are plants, and its up to you to decide on what you like which is main factor. Also, it depends on lighting you have considering some plants require much more light then others. But to have it be a 'well-planted' tank, I would suggest fast growers over slow growers (less plants to buy, providing more bulk as they grow in).
2. Decorations to me depend on the inhabitants and your personal preference. Driftwood I think ALWAYS looks nice in planted tanks, and a few rocks break the monotony of an all driftwood tank. Properly placed they can be good for growing mosses and the like on, at which point you wont even see the rock if you let the moss overtake it as most will.
3. Soil Master Select Charcoal is the only one I have experience with. It is dark but not extremely dark. Eco-complete is much darker from what I have seen in the stores.
4. As far as using the filter for C02 diffusion, I have never done but if you choose not to do this, a powerhead works nicely. Put c02 output on the intake of powerhead in a corner of the tank and let it do its thing. As far as your question though, your method sounds about right. You just want the c02 to get the most mixing, (smaller bubbles =more c02 diffusion as I am sure you know). You might not have to splice your intake though, you could always stick end of the c02 output into the intake and secure it. That way if you choose to go a different route down the road you wont have to patch your splice.
A LFS has a 90 gallon corner planted tank that is beautiful. All the corner tanks I have seen typically have a nice centerpiece of driftwood/rock formation with plants around. Taller stem and root plants the grow upward sit behind it providing a nice backdrop, and then youve got your grasses in the front (totally covering front, and/or possibly covering the left corner and leaving right corner un-scaped) or even covering from left and right corners with grass and leaving a "walk-way" of substrate from the middle area leading to your main centerpiece of driftwood or rock.
But keep in mind, in the end every tank is different and even the most beautiful tanks from ADA Aquascaping competition, I can always find things that I would have planted and aqua-scaped differently. Therefore, it all boils down to what YOU like.
I would think in a corner tank, with a rounded front, 1 flat back wall of tank covered in moss would look nice, with some tall stem/root plants on the other side in which they will grow in and "meet" in the middle creating a forest look on one side, and a over-grown mossy look on the other side. A nice flowering or colorful plant on left or right side of the centerpiece of wood/rock would always look nice as well.
Best of luck with your tank, please post pictures I would love to see. Its always exciting to setup a new tank, Ive got a 75 gallon and a 29 gallon I am working on right now!!