I've never had a problem adding new fish, provided that I don't add too many. And yes, there will be algae. Algae is actually a sign of a healthy tank....but not a sign of a pretty one. And it's not really the plants that cause the algae. It's the lack of cleaning and water changes. But it is important, if you are not doing water changes to not add too many fish, or to have plenty of plants. It is a weird balance that took me a while to achieve with my garden. For starters, it takes about 3 weeks for the ammonia to bring the nitrites and then the nitrates. Your end goal is to have nitrates in the water. They get rid of the nitrites (the nitrites get rid of the ammonia) and are what the plants use. If you are able to get water from a tank that already has nitrates, then you can cut that time significantly. You also need to have gravel or something like bioballs, or, in mine, where the water goes thru a 50 gallon drum before going into the tubes that are holding the plants, I have a ton of window screen (not the metal kind). You need something like that to increase surface area for the bacteria to grow on. The more bacteria (nitrites and nitrates), the more fish you can have. The more fish that you have, the more plants that you need.