They are both correct, but as they seem to focus on the plants, it makes more sense to have plants as the subject of the sentence, i.e. the first is more natural. If we are focusing on the fountain, it makes more sense for it to be the subject: I saw a fountain that was surrounded by plants.

Both are correct and natural, it just depends on what word you want to stress importance. The first sentence is written in the active voice and the word 'plants' is the subject that is doing the action. The second sentence is written in the passive voice and we use this form when we want to focus attention on the person or thing affected by the action, in this case the fountain.

They are both fine.

The passive voice is better here ( A fountain is surrounded by some plants.)