hole or growth on a beta

Question
I have one betta in a 5 gallon hexagon, bio-wheel tank, with filter and heater (78-80 degrees).  I change about 20% of the water every 2 weeks.  I feed the betta two pellets twice a day.

He's always been fine but today I noticed either a growth or hole on the top of his head behind his left eye.  Through the glass it looked like a crystal or clear growth.  But when I opened the top, it looks more like an absence of skin or a hole.  It's really hard to tell.  I've noticed more and more algae growing in the tank recently.  I wipe it out with a scrubber when I change the water.  But now I see it in the gravel as well.

Answer
Test your water - you're bound to have a build up of nitrites or nitrates. You need to start changing out more water more often. About 20 percent twice a month is not nearly enough. You can put in about 5-6 drops of Prime to help detoxify ammonia and nitrates/nitrites each time you do a water change, until your tests show a non toxic level. If you don't have one of these tests, take a sample to your local pet store to have them test it for you - most stores will do this for free. Or if you'd prefer to do it at home, they do sell them at most pet stores as well. While I personally prefer the strips, as they are quicker and easier, some people will argue that the liquid tests are more accurate. I've never heard conclusive evidence either way. Be sure when/if you buy the tests, you get it for ammonia, nitrites, AND nitrates.
As your water quality become less toxic, you should notice improvement in your fish's condition. You may need to add medication once you've done a few water changes though (I recommend 30 % once a week) and he does not seem to be doing any better.
Hope this has been helpful!