Algae problems are most often caused by excessive nutrients in the water. Extra nutrients can build up in many places.
- Canister filters if not cleaned frequently can build these up.
- If you have any filter pads in your system, they can also trap nutrients.
- A combination of porus rock and inadequate flow around those rocks can lead to nutrient spikes.
- Overfeeding, as you've mentioned leaves uneaten food to get trapped and decay.
- A death in your tank without a body removed is another source.
- Older light bulbs that are producing a different spectrum of light can lead to algae outbreaks too.
- High dissolved solids in your source water can also be a culprit. You should have your pet store do a TDS test before you take your next batch of water home. Ideally, its best to invest in an RO/DI purification system of your own so you can control this crutial element of your tank's health.
Your description sounds like you may have an outbreak of green hair algae. I am not personally a fan of using chemicals in our tanks, as it can throw your entire chemistry out of wack very quickly and cause more problems than in the first place. Besides, those won't do you much good if you are not locating and dealing with the source of your excessive nutrients.
Once you've located the nutrients and taken steps to reduce or remove them, you then need to tackle the algae itself, and you'll need to be aggressive. First, you need to remove as much as possible manually. Keep in mind that if you take it off your rocks, remove it from the water too! Otherwise, the nutrients that the algae has stored up will be released again back into the water as it dies.
Up to you how you want to do this, but I would reccomend during your next water change, (and if you can solicit some help with this, it will go easier) use a siphon hose to try to suck up as much of the algae as possible. This will have the added benefit of "vacuuming" out any loose algae spores that may float free if you remove by hand.
Once you've finished siphoning, and only if this is feasible in your situation, try to remove the worst rocks from your tank. Place them in a bucket of waste SALT water that you've removed from your tank. In a second bucket, you should have clean SALT water that you've either mixed yourself, or that you recently purchased from your pet store. For this task, you will also need a turkey baster, a new toothbrush, and a new dish brush. Do not use any of these items that have already been used for their manufactured purposes!
Take your algae-infested rock and hold it underwater. With either of the brushes, scrub the rocks vigorously, paying special attention to smaller nooks and crannies in the rock. Fill the turkey baster with some CLEAN salt water and squirt it into the smaller areas. Try to remove as much algae as you can. Once you're done, hold the rock above the water and use the turkey baster to rinse off the entire rock, then place the rock in the CLEAN salt water bucket. Repeat this process for as many rocks as you can, then put the rocks back in the tank. Finish your water change, then leave your lights out for 3-4 days. Do another water change at the end of this time to remove the nutrients left by the dying algae. If your light bulb is old, you should change it before turning your lights back on.
Hopefully some of these steps will help you. You may have to repeat the entire process before you can completely remove the algae from your tank.
