A few things I can offer, to start you off.
You probably won't be able to keep many angels in a 30g tank, once they get full-sized. Angel people can probably give you a better guess, but I'd probably stay with 2, maybe 3 tops.
From everything I've read, discus take a lot of maintenance. Might want to read up on them before diving in.
Most cichlids are aggressive. Ususally, they're kept in 'cichlid-only' tanks, from what I understand.
Cories will not eat algae. They do an excellent job of cleaning up leftover food on the bottom of the tank, though. And, they're fun to watch.
A good 'cleanup crew' is an important part of a tank, in my opinion.
Filters: Most people do not like UGFs (Under Gravel Filters). I would probably recommend what's called a HOB (Hang On the Back -- of the tank) type of filter. Some popular ones are Penguins and Emperors (made by Marineland) and AquaClears (made by Hagen).
The way filters are 'measured' is usually by their flow rate, in gallons per hour. You divide the gallons per hour rating of the filter, by the total volume of your tank, to get an estimate of how many 'turnovers' you'd get per hour. The recommended number of turnovers is 5 (some aquarists will say up to 10). So, for a 30 gallon tank, to get 5 turnovers an hour, you'd want a filter that is rated at 150 gph. In the case of the two brands listed above, I would suggest a Penguin 170 or an AquaClear 200.
Thermometer: Get a glass thermometer, don't rely on the 'stick-on' plastic type, they're not too reliable. Also, realize that sometimes thermometers are 'off', so it helps to check it with one that you know is correct. For instance, I know that in one of my tanks, when the thermometer reads '80', it's really 78 degrees. Glass thermometers, with a suction cup to attach them to the tank, are usually only about $2 US.
Heater: The usual rule of thumb is '5 watts of heating per gallon'. How much heat you'll need does depend on the temperature difference between your tank water and your house (at its coldest), but I think a 150-watt heater should do you fine (that
should handle temperature differences up to about 12 C) Also, I personally would recommend a fully submersible heater.
Personally, I like real plants. I think they make a tank so much nicer, and benefit the fish, benefit your water quality, and (important -->)
if you pick the correct plants for your situation, they require minimal extra work or cost. Research some easy to grow, low-light requirement plants. Some of these will include java fern, java moss, anubias, hygrophilia polysperma, hornwort, 'crypts', and anacharis.
Substrate (gravel) color is a matter of personal taste, but personally, I find that brightly colored gravel looks odd with plants.
And, lastly, in my opinion, every minute of research you do beforehand is worth many minutes afterward.
Have fun.
I'm not a big fan of tall tanks because it's harder to get way down there to
