Hi Juno164, Yes this is safe for a betta in a 7G tank. I would recommend the small size. Thank you.

This size stone is kind of big and powerful. I use it in my 55 gallon cichlid tank and they love it. I would probably suggest a smaller size stone 

Hi Styx, Links are below for an airpump and airline tubing. We recommend the black airline tubing if its will be outside as the clear can cause algae in the line when exposed to sunlight. https://www.petco.com/shop/en/petcostore/product/imagitarium-air-pump https://www.petco.com/shop/en/petcostore/product/imagitarium-black-airline-tubing Thanks,

Depends on what type of fish and your pump size. But a small sized ayirstone would be sufficient.

Hi TetraTime, We would recommend either the medium or large size. Thanks.

Hi, I have got the large one, not the small one, but the small one is only 1.5 inches which is extremely tiny if you look at a ruler. I'm unsure what the Petco volcano is, but I would suggest to look on the items page to see if it shows hows how big the volcano's opening is. Most likely there will be a hole in the back for the airstone so I do not see why it would not fit in there being that it is small. You could also look on youtube to see if someone has a review on the volcano. That could help you as well. As for air tubing, no, you have to buy that separately. Rember that you will need the airline tubing, and a check valve as well as the nob to control the amount of air that is coming out just in case you do not have them already. Best of luck! :)

It does a little bit but my pump is adjustable so I can control the bubble flow

I've been using the Tetra Whisper for 10 gallon tanks, and it puts out a mild amount of bubbles. The surface of the water isn't violent.

Do you have a check valve?  If so, make sure it is right side up.  Had mine installed wrong and had the same problem .  Hope this helps.

no, it will not harm any fish, it actually make's there fins stronger!