While there are vegan alternatives to a lot of the food for our companion animals, they can be hard to find and don't always fulfill the appetite. What I mean is this...of our two dogs, one will eat just about any food we give her, but the other is pretty picky. We have yet to find a vegetarian/
vegan food that he will eat, even when we dress it up with olive oil or some other flavor that he likes. The alternative is to give him meat.
The problem with meat is then it's hard to find companies that have good practices in the preparation of the meat. There are all kinds of by-products added, and some of the major brands even have horrible practices where they use meat from animals that should never go into production...such as using dog meat to produce dog food. It's horrible.
On a different note, I am not trying to judge anyone on this thread at all...but I have something for you to consider. Basically, adopting and caring for domesticated animals is a good thing, because these animals (such as dogs) are so domesticated they could never survive on their own in the wild. They have become dependent on us. However, many "pets" are not domesticated, and they are meant to be in their natural environment. I am not suggesting anyone change what they are doing...at least these animals are being cared for, and that is wonderful. However, it is something to consider for the future.
But, back to your question...algae is great...but yeah, I wouldn't worry too much about what you feed them. Just take good care of them like we already know you will!