Help!

[The betta expert]

Hi Everyone,

My betta fish Delta,who I just got has a problem. This afternoon, when I went to check on him, I noticed there was alge in the tank. I was like oh my gosh! So, I cleaned out my tank thourghly, I scrubbed the sides for a half hour, and then I decided to take all the gravel out, so I could rinse it properly. Usually, I just rinse it out. So, anyway, at the bottem of my tank, I found these little black thingy's. They basiccly looked like his food( bettabites), exept they were black. I rinsed some water around in them, and the water turned brown. I may just not know, and the anser is really ovius, but um, what are those things doing in my tank and what are they?
Here's some info on my tank.

Size: 2.5 gallions. Delta likes a small tank, I think he feels scared or somthing in a larger tank.
Temp. Usually about 80 degrees F. ( 27 degrees C.)
Food: BettaBites
Water Conditioner: Betta Basic
Stuff: a bunch of plants, a no fishing sighn, a heater, and a thermomater.

Please reply, guys. By the way, I'm Lia.

Thanks!