Recently decided to get my little one a pet fish. I selected a betta, one that was very aggressive and active initially. I placed him into a five gallon tank with a filter and an LED light. He has an adequate amount of plants and hiding places, I make sure the temperature is between the appropriate ranges, and do my best to make sure he has a night and day cycle. After about 1.5 weeks I noticed he started making his first bubble nest. I was very excited and happy to see this, however, it's been roughly a week and I've noticed a slight change in his behavior. I'm no expert on fish behavior so I'm not sure if I'm overly concerned or if there's something going on. His appetite has decreased quite a bit. I was giving him between 4-6 pellets a day after I noticed him coming to the surface more frequently in search for food when giving him just 2-3. I've cut back on his food since he's not eating as frequently, after giving him 2 pellets he just doesn't seem that interested in eating. He still seems to be pretty active, no unusual signs aside from his appetite. Does he just not enjoy the food or should I suspect something more?

Could there be rotting food on the bottom? Is the water cloudy? I'm wondering if you overfed him.
Does the filter create bubbles to oxygenate the water?
Fish don't always gulp at the surface looking for food.

There are a few pebbles floating around the tank, the water is clear otherwise (the tank has only been set up about 2 or 3 weeks). I did over fill it a bit, though, so the filter doesn't create bubbles when it flows into the tank. This hasn't seemed to bother him much in the last few weeks, though. Should I remove some of the water? I've thought about removing some and possibly adding a bit of warmer water to boost the temperature a couple degrees. The water is in the low 70s, which I read was okay for a betta fish, but I think it's dropped a degree in the last week.

I'm also wondering (as I've read other's experiences with Bettas) if maybe he doesn't like the pebbles? He didn't seem to mind them in the beginning, but I read Bettas also enjoy insects and brine shrimp? I'm not sure what the pet store fed him, but I don't believe it was anything more than what I feed him.

I'd let some bubbles air the water, and buy food just for Bettas, such as brine shrimp.
Some pet stores are good, others take the cheapest way out, hoping to sell before the pet gets sick.

The tank is to big. I had mine in about a gallon tank with no problem, pebbles and one of those little fake trees. He liked hiding in it, doesn't need the water oxygenated, just changed when it gets cloudy. They are extremely easy to contain in a large cup of water while you change but I had two small tanks for that purpose.

They are a very friendly fish, mine waited for me to appear in the morning.

And yes, you are probably feeding too much. Read the labels.

Ok, so you have a 5 gallon tank and a filter...this is good. The tank could be smaller but a Betta will do fine in any size, it's just a waste to go too big.

I have had many Bettas over the years and, to be honest, I never bought into the specialized Betta food or pellets. They always got what I fed my other fish....TetraMin tropical fish flakes. They did fine with this. Is that the answer to your question though? No, I don't think so. I know plenty of people who swear by the pellets so you are probably ok there. As a general rule of thumb, feeding amounts work like this: Feed them what they will eat in 5-10 minutes. You don't want any food left floating around after 10 minutes. If it is, you are feeding too much. If it is eaten up too quickly, obviously you are not feeding enough.

Your temperature sounds about right though they generally like it a little warmer, closer to 80. You have to keep in mind where these fish originate.

Now, to your problem...sometimes fish go through things just like people and they may act strange for a bit or may not want to eat...but when it happens, it is a good idea to check a few things. Being that you have only had this fish for 2 or 3 weeks, I am wondering about the water quality. You may want to take a water sample to the pet store and have them test for acidity and PH as most pet stores will do this for free. I think this is where you will find a problem in your set-up...if, in fact, you do have an actual problem. I am not going to write all that I know about water chemistry for fish but I did find you a pretty good explanation about it and what to look for.
betta water

Good luck and let us know how it all works out.