Is my puffer fish ok?

Question
I just recently bought three puffer fish and I am starting to wonder if I did the right thing. I've been reading and starting to realize that shortly I will have to upgrade my tank...but they are very tiny right now so the 10gallon i have should be fine...right? My question(s): First, the fish seem to change color, is that normal? I had three, and then one of them died, so i returned it and brought a new one home. One of the original three was fine, but the other one seemed affected by the death? It has turned a deep dark green and the underside of him is gray/black speckled. He hovers behind the plants or near the bottom in the dark, he's not social and I haven't seen him eat since the death of the other fish. Is something wrong with him? Also, I don't know the first thing about the water necessities. We have a super huge filter, that seems to keep the water clear. We have live plants, they seem to be fine in the water as well, and I use these little tablets that fizz and are supposed to "cleanse" the water when I change the water. Also I have some type of "aquarium salt" that says add one tbs for every 5 gallons of water. Do I need to add anything else? Is that ok enough salt? I read about all sorts of nitrogen and oh, what is cycling a tank? Yeah, I'm really new at this.

One more thing, I also have one of those "beta bowls" I bought a vase and then some plants that just sit at the top and the beta lives underneath. does it really eat the roots or should I be feeding it something else? Well, thanks in advance, any help on the matter would be greatly appreciated.

Answer
Hiya!

Ok well to start off I have no idea about Puffer Fish so I can't help you there. I do know that you can get both marine and freshwater puffer fish and I'm going to presume that yours are of the freshwater variety.

Usually people only add aquarium salt if they need to eg for medication.....

Cycling a tank is where your tank goes through its 'cycle' to set up all the bacteria etc your tank needs to thrive. When you first get a tank, you usually add one or two 'expendable' fish such as goldfish and use them to add the ammonia to your tank which will cycle it. This process can take between 4 - 6 weeks an ideally no other fish should be added until your tank has been cycled and all your water parameters checked.

Your Betta does NOT eat roots...you need to be feeding him either pellets that are designed especially for him or freeze dried blood worms also work well.

Hope this helps!

Cheers! Rach