Also check your tank for a dead snail or hermit crab. That can foul a small tank very quickly.
I would also suggest throwing on some sort of mechanical filtration and do some serious turkey basting of rock and sandbed. If you have never basted your tank before, be ready for serious cloudiness. Don't worry about the cloudiness, it will not harm your tank. Have done this proceedure in a couple of tanks with sandbed and cyano problems and it is safer than using chemical cures, imo.
Anyway, once the once the tank clears, remove and clean the media in the mechanical filter, then baste the rock some more. Also gently basted the sandbed where the cyano grows, not so much as to cause a sand storm, but enough to loosen the cyano and some of the detritus into the water column. When the tank clears again, clean the filter media again. Your skimmer may also need the collection cup cleaned more frequently during basting, so keep an eye on it as well.
Do this basting and filter media cleaning thing at least once a day for as long as the tank gets cloudy during basting. By the end of the basting period, you will have exported tons of detritus which helped fuel your algae problems.
Once you get things under control with the large water changes, continue basting and cleaning filter media in your tank before doing weekly water changes of at least 15%.
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