I'm going water with my bard (That's right, I don't mind much about healing, that's what the Blood vamps and devine priests are for ) and I was wondering what pet should I use when I reach lvl 30. (Wind puppet, Chained demon, Nightmare, Echo Fruit or Scarab ) Anyone have an advice for me on the pets?

I always liked pets with high attack (i.e. Wind puppet, Khonem, Nightmare for level 30). I've never had an Ecto Fruit so I can't comment on those. If you have the funds, you can try to get an AOE skill + Mana drain onto the pet. There is a risk of failure though, so make sure it doesn't break your bank. Attack pets have high attack, so Brute Force (atk buff) work the best on these.

Tanky pets (Chained Demon, Scarab, etc) are useful if you plan on getting Invigorate, the hp buff, or Iron Bones, the def buff. Since tanky pets get more hp and def gain, they will make these buffs more effective.

In the end, it doesn't really matter what you get. I have an attack pet with AOE + mana drain, but stuck Invigorate on it for the hp. Whatever floats your boat :P

You will most likely swap out your pets at higher lvls anyway. So don't put too much money into them now.