1) Sand and Eco, probably not a good idea. Fine substrates will always sink to the bottom eventually. Unless you are capping dirt with sand, I wouldn't use sand to cap anything else in a planted tank.
2)see number one.
3) You could get away with 2-3 inches of sand as a main substrate. If you know what your doing, you can pull of a fresh water deep sand bed just as the salties do, but I wouldn't recommend it for a beginner. Also, if you want 5-6 for hills or what not, use lava rock, egg crate or something similar to bulk up your terrain. This keeps the good bacteria thriving and allow you to use less substrate. Now if you have some heavy root feeding plants, you can pull off deeper substrate beds as well.

I recommend doing a bit of search on here for others that have done sand beds. I have not done them personally yet, but I have done enough research and seen enough post to know what I would attempt and what I won't. My words are just opinion for you will need to put together information yourself so that you can form your own opinion.

 

Thanks for the advice. I'm having a rough time making a decision on substrate. I just want optimal conditions for carpeting and therefore want a topper with small granuoles. However its spelled.

I'm really open to ideas if anyone has any

My Finnex Ray 2 just arrived today. According to Hoppy's graph, http://www.plantedtank.net/forums/10-lighting/184368-lighting-aquarium-par-instead-watts.html, I should be above 60 par (high light) in the majority of my tank at 18 inches deep. Along with my pressurized co2 setup I'm hoping to be in pretty descent shape for carpeting....

I just need to figure out the ideal substrate!!!!! Can I use a sand topper with MGO?

 

Dirt your tank. Get some MGO or something similar and then cap it with the sand of your choice. Otherwise you will need to supplement the sand with root tabs to get a lush carpet.

 

I think I may want to go this route. How thick of a base should I go on the MGO? On the sand?

Not that I would, but could I have consequences if I were to lay 3 inches of a MGO product and top it with 3 inches of sand? I read that 1 inch of "dirt" is all a tank should have. I'm just trying to get a better understanding.

Also, will many carpeting plants grow roots through 3 inches of sand and into the MGO, or do they tend to stay in the sand and just obtain nutrients released into the water from the mgo? This may be a dumb question, but I havent gotten into learning much about the plant aspect of aquascaping yet. Just trying to get the lighting, substrate, and co2 down first. LOL.



Bump:

I capped Eco-complete with CaribSea Peace River Gravel. As one plants or otherwise disturb the substrate it is difficult to keep some of the Eco from coming to the surface. However, it isn't a huge problem. I do use root tabs.

I think next time I might just use Peace River and root tabs.

Some people use Eco without a cap. It is a rough volcanic rock that could be hard on Cory cats. I've heard mixed reports, but it looks pretty abrasive to me.

Where you want 5 or 6" of substrate, it might be a good idea to put something in there as a spacer. Deep substrate can develop anaerobic pockets that produce methane. Use inert rocks such as granite for the added height. You might be able to get broken pieces of granite counter top from a place that does custom work. Don't use marble.

Thanks for the advice. I was thinking about using different sized tupperware and filling them with the gravel I have laying around the house. I also read of a few other people with the same idea. I want to use that as a base for my "hills.".

 

I capped Eco-complete with CaribSea Peace River Gravel. As one plants or otherwise disturb the substrate it is difficult to keep some of the Eco from coming to the surface. However, it isn't a huge problem. I do use root tabs.

I think next time I might just use Peace River and root tabs.

Some people use Eco without a cap. It is a rough volcanic rock that could be hard on Cory cats. I've heard mixed reports, but it looks pretty abrasive to me.

Where you want 5 or 6" of substrate, it might be a good idea to put something in there as a spacer. Deep substrate can develop anaerobic pockets that produce methane. Use inert rocks such as granite for the added height. You might be able to get broken pieces of granite counter top from a place that does custom work. Don't use marble.