Possible sick betta? :(

[Thisbinaryuniverse]

Hi guys, I think my betta is sick. I got my first betta 2 weeks ago from petsmart, and he died within 2 days, so I went out and got another betta from a real fish store last week. He seemed perfectly fine until a few days ago he would randomly twitch while swimming, so i called the store and they told me if he continues to give them a call. Well, he seemed to have stopped doing it, but in the last few hours especially, he is seeming more and more lethargic, keeping his fins close to his body, and just laying on the bottom of the tank, which he never seemed to do since I got him. I can't see any signs of ick or fin rot, but I don't think I've had him long enough to really know how he normally looks. The store is closed until morning, so I was hoping someone here could help me with what I should do, I'm really concerned especially after already having one dead betta. :/

ousing
What size is your tank? 3 gallon
What temperature is your tank? 76 degrees
Does your tank have a filter?no
Does your tank have an air stone or other type of aeration? yes
Is your tank heated? yes
What tank mates does your betta fish live with? none

Food
What type of food do you feed your betta fish? freeze dried bloodworms
How often do you feed your betta fish? once or twice a day

Maintenance
How often do you perform a water change? once a week
What percentage of the water do you change when you perform a water change? 25%
What type of additives do you add to the water when you perform a water change? none

Water Parameters:
Have you tested your water? If so, what are the following parameters?
No
Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? not at all, as far as i can tell
How has your betta fish's behavior changed? lethargic, twitching, laying at bottom of the tank
When did you start noticing the symptoms? earlier today
Have you started treating your fish? If so, how? no
Does your fish have any history of being ill? bio
How old is your fish (approximately)?

[Shirleythebetta]

do you use a water conditioner?

"What type of additives do you add to the water when you perform a water change? none"

[Thisbinaryuniverse]

Oops, yes i did, some betta "Waterconditioner/dechlorinator."
Update, though. I went to clean his tank last night and discovered a dead spider in the gravel, not sure how that got in there because his tank is covered. So I did a full water change, and added a 1/3 of a teaspoon of aquarium salt to the water, along with the water conditioner. I also added some new rocks to his tank, because the gravel I had in there was kind of ugly anyway. I woke up today, he looks a bit better, but there is some white gunk on a few of the rocks. Anything to be worried about? I washed the rocks before I put them in the tank, they are aquarium rocks as well.

[Shirleythebetta]

That could be a few things. Try to PM OldFishLady or Sakura8 and tell them your worries. They are great with this kind of thing.