Join Date: Feb 2009
Location: Northern California
The top soil may or may not be ok. If it has any kind of fertilizer added to it I would get rid of it and start over as they can often be toxic to fish. If it was an organic soil it should be ok, but it will probably continue to cloud the water unless you cap it with something such as sand or gravel. There are lots of posts in the substrate section of the forum about how to make mineralized topsoil, which should not cloud the water and will be safe for fish. As long as you still have no fish, a 100% water change will not hurt anything.
Depending on your water and the types of plants you want to grow, dechlorinated tap water should be fine. Personally, that's what I use, but it does limit my plant choices as I have very hard tap water and many plants prefer soft water (such as Hygro species). You might wish to research the plants you want to use and if they will do ok in your local water, if not either use buffers or RO but if you go RO make sure to use ferts.
Carbon is absolutely not necessary in a planted tank. Just make sure to do water changes regularly, but the plants will take up a lot on their own. I do a roughly 30% wc on my two 10gal. and one 5.5gal. every week and use no carbon, just filter floss and lots of bio media and they have been going for 2yrs, 1yr, and 1mo. respectively.
As far as fish stocking, its pretty subjective. If you plant heavily and do frequent water changes you can have way more fish than is usually recommended (1in of fish per gallon of water- actual water in the tank). I would never recommend only 2 tetras though, as they are schooling fish that will be stressed in less than a group of 4 and are way more likely to get diseased and die. I would just do the oto and the guppies, you could probably have 10 guppies comfortably in the setting you described and they are very entertaining on their own!
As far as plants, I would start with cheap ones and make sure they like your water parameters, but other than that just do what you want as almost all of them are quite hardy with your lighting, especially if you use Excel and/or CO2. Other ferts I would recommend are trace elements (with each water change), Iron (small doses 3x a week, not on the same day as the trace), and Potassium (same as Iron).
Good luck on your new tank, but just a warning: planted tanks are VERY ADDICTING! You should definitely post pictures when its set up, it sounds like it's going to be very cool with the bog and aquatic plants together!