Congrats on the 90G! =) You have a couple of questions that cant be answered until youre a little more specific on the type of set up you are going to build =) will it be a "REAL" terrarium? with background, vines, water falls and such? or one of those so called "terrariums" that are basically a fish tank with plants....pots and all just sitting on the bottom of the tank? (I call that a mini green house!) no offense to anyone, I just dont think that last mentioned set up is a terrarium, please let us know the specifics =)

The thing you mentioned is actually an undergravel filter and is meant for filtering a fish tank by pulling dirty water and debris down through the gravel while fresh water is returned to the tank through uplift tubes that would have been attached to the upraised openings along the back of the filter.

You could leave it in there for drainage but you would need to put some type of liner over it such as a layer of pea gravel or better yet, landscape fabric to keep the soil from washing down under it and eventually filling it in and stopping it from draining. I would do a layer of landscape fabric topped by a one inch layer of pea gravel and then build up your soil base from there. Be sure to plug the holes where the uplift tubes would have been as it appears at least one of them is missing its cap. The big advantage of re-using the undergravel filter is it's very light and if properly covered to prevent soil from infiltrating beneath it, it will provide good drainage and require less fill in the tank.

Another thing you need to consider for the tank is the lighting. For a tank that size, I'd recommend compact flourescent lighting or a double or triple flourescent hood. The hood will fit the tank perfectly these types of light fixtures will provide ample light for the terrarium. With the double and triple lights, you can even add special bulbs that will simulate moonlight and such to give your plants as close to a natural experience as possible. A good place to look for them is drsfosterandsmith.com. They have really good prices and outstanding customer service. I've been doing business with them for years. You can talk to one of their technicians and they should be able to give you lighting and hood options for the type of terrarium you want to set up. The hood and lighting will probably cost a bit but you got such a great deal on the tank and stand that I think the extra money spend on the hood and lighting will be well worth it.

Mine has worked great leaving the UG under gravel filter in place and then putting rocks on top of that and a very loose soil. In terrr soil can get pretty tight unless you put something in it like sphagnum.

I don't have a water fall in mine though. I found that it was just too wet. All of my water came out of the waterfall and ended up drowning the plants.

Best of luck to you!

Lisa