Beta death

Question
My 2-year old Beta died suddenly without outward sign of disease.  I kept him in a 5 gallon Marineland hex tank with filter, live plants and a full spectrum light bulb.  The water had no ammonia in it (I tested).  I wasn't changing the 20% of water very often, maybe only once a month at filter change.  But it was such a large tank for a small fellow.  On Friday, he was at the bottom of the tank but moved when the net touched him.  I changed out 20% of the water, and since it was going to be a warm weekend (I kept him at work), I left the lamp off.  When I came in Monday, he was dead.  I miss the little guy.  The plants had built up a lot of algae which was getting hard to get out.  Does one need to do 20% water changes weekly for a tank that size, or is monthly okay?  I'm trying to determine if I did something wrong, so I don't repeat it with a new fish.  I'll have to replace the plants, gravel and filter for a new fish after cleaning the tank, right?  Thank you

Answer
Carol,
I'm sorry about the loss of your betta fish. It is easy to get so attached to them. The most likely cause is nitrogen accumulation, which will happen when there are not enough water changes. The smaller the tank, the more you have to clean it. A five gallon may seem like a lot of room for one fish, but they eat and poop a lot over the course of one month - and while you may not have had any ammonia there is a very real cause of poisoning from nitrates alone.
Next time, just make sure to do your weekly water changes, and keep an eye on your nitrites/nitrates as well as ammonia. Gravel vaccuuming is a great way to do water changes- it's the only way I EVER do them!