As fluttermoth said, algae is normal in any aquarium. But there are some steps we need to take to keep it in balance. If you have live plants, algae is much more of a concern because it will coat the leaves and slowly kill them and the plant. But if no live plants, algae is then more of a personal issue, within reason.
Algae will appear with nutrients (fish provide these) and light. There are many types of algae. It is not really "bad" because it uses the nutrients to produce oxygen, though this is limited.
The front glass should be cleaned inside at every water change whether you see anything on it or not. All surfaces covered by water will naturally develop a biofilm, and adhering to this will be various bacteria, microscopic critters, and algae. The front glass should be kept clean so you can see inside the aquarium, so run a sponge type scraper or use a clean (fish use only) sponge with your hand, over the front glass and the side glass if you like.
As for the decor, that is up to you. Controlling light (intensity is important, as well as duration) and nutrient availability is key to control of algae. Don't overfeed, don't overstock, have light that is not too bright, and not on for longer than a few hours or some regular schedule.
First thing you need to do is increase the water changes; every two weeks is not sufficient, and part of the problem. Once every week, and change about 1/2 the tank volume. Clean the substrate at each (use one of those water changers that allow you to clean into the substrate without pulling out the substrate). Feed minimally, even missing a day or two a week won't hurt. Do not feed prior to a water change.
If you provide some data on your tank light we may be able to suggest something there. You should be able to have it on for several hours, maybe 6-8, provided it is suitable.
You have some issues with fish, as fluttermoth alluded to. Swordtails are too big for a 75 liter (20 gallon) tank so this will be a problem as they grow. Tetras are shoaling fish that must have a group of their own species, 6 is usually minimum, but you don't have space for all these, so you should consider removing (returning?) some to sort this out. Black widow (skirt) tetra are really too large for this tank too, and they can get fin nippy in cramped quarters or with fewer than 6.
You should have a test for nitrate. Many of us recommend the API liquid Master Combo test kit; it has tests for pH, ammonia, nitrite and nitrate. Both pH and nitrate are worth testing periodically, and before a water change is a good time, as they can alert you to problems.
Byron.
