Hi gingerinaustin, I suggest that you check out this link in which Tom lays out the steps to a successful low light, low tech, non-c02 tank. Things need not be all that complicated and quite honestly, doing a low tech, non-c02, low light maintenance tank right from scratch rather than spending countless months trying to fix a tank gone wrong, may be a better option. There comes a point, where you have to decide if it is better just to tear a tank down and start from scratch start fresh to avoid with dealing all the issues not to mention of pocketing additional expense due to having to double dose or triple dose things such as excel for months on in to fix the problem.

Here is Tom's Link and suggestion for setting up a low tech, non-co2, low light tank. I have had great success with it and it is something certainly worth looking into.
Non CO2 methods

Personally, I would not go more than one month without a water change, unless you are running a UV sterilizer. And not to confuse matters anymore, but there are people (e.g., Diana Walstead) who have kept minimal algae(which mostly served as food for algae eaters and did not overtake a tank) tanks without water changes for 3 months or longer. I think that it all comes down to achieving a balance and maintaining it whether you choose the high tech or low tech road. Water changes in my opinion cannot really hurt as long as you add some ferts, assuming a low stock level of fish, after the water change, especially in the case of a low tech tank, at least that is my experience.

Good Luck.