There is a lot of problem algae going on there in those photos. Fortunately all is due to the same thing, an imbalance of the light and nutrients. When algae increases to the extent it becomes a problem (nuisance, killing plants, etc) it is always due to this one factor, an imbalance.
Light drives photosynthesis, and each plant species has a minimum intensity requirement; intensiity is one aspect, another is spectrum, and the third is duration. An insufficiency is any one of these cannot be corrected by increasing the others, and vice versa. The intensity/spectrum is likely not the issue here in the sense of being inadequate, but the intensity may be too strong. A 20g long tank is shallow, so more light is reaching down into the water than would be the case with higher tanks (which is why they generally require brighter lighting) so here reducing the duration might help; you haven't indicated the duration, so it is difficult to suggest changes that might help. Floating plants also help this a lot.
Nutrients are the second aspect of the balance. Plants can only use the light fully for photosynthesis if all 17 required nutrients are present. As soon as one is insufficient, photosynthesis will slow, and may stop altogether. In such cases, algae is quick to take advantage because it can use any light and fewer or imbalanced nutrients. I have had problem algae increase from too much light (either the intensity or the duration), and from too little (intensity), from too many nutrients and from insufficient. Fish food and water changes provide nutrients, and obviously any plant additives.
Knowing the daily duration of the light and if any fertilizers are being added will help us.
The light is on a timer from 7am to 530pm. I have floating plants but this is where the algae started. (reference the photo with the Cory in it) I also now have quite a bit of duckweed (came with the corys). I turned the time down from what I had it when I had my fish in the 10gal tank with the included hood/light.
I am not currently adding fertilizer. I stopped three water changes ago. I used API leaf zone. Aside from the floating plants, my amazon swords are planted in pots in the aquarium with a half a root tab.
Aside from the fertilizer, I’ve not changed my care routine and that’s why I figure the filter (maybe too small), the new plants (had algae when purchased.??) and light is the issue.
My aquarium store told me this filter should be enough, but I’m not convinced. I’m looking into a canister filter that can manage more than 20gal.
I pulled out all the floating plants and separated the parts covered with the stringy algae. I also cleaned the fake trees with a toothbrush. Seems now the algae is more like a mossy covering over everything and if I touch it, it disintegrates and floats around. Also lots is dislodged when I shake the large floating plant.
I buy my water from the aquarium store. They RO filter their own water and sell it by the gallon. They added ‘stuff’ back into the water the first time I bought it, but have not since.
The pH is about 6.5, GH between 0-30, KH 0, Nitrate 0, nitrite 0 According to API 5 IN 1 test strips. The aquarium water reads the same except nitrates are between 0-20 and today is water change day.
I feed my bettas pellets and the Cory and platys sinking pellets. 1/2 Algae wafer weekly. i have 2 female betta, 2 juvenile platys, 3 Cory cats, 2 mystery snails, 3 nerite snails, And 2 cherry shrimp.
That's 10 1/2 hours with the lights on. My tanks (different in each tank) typically get 7-8 hours a day.
It would be worse without the floating plants and...
the top of the tank has the most light - which suggests that too much light is a factor
I agree, the light is likely the main issue here. Reduce it down as seangee suggested; my tanks have 7 hours of tank light each day, and on a timer so it is consistent (this can help too, and fish benefit).
This might be cyanobacteria, or it might be organics. Can you post a clear photo of just this stuff?
I would stop the API Leaf Zone. This is iron and potassim, nothing else, and excess iron can cause problem algae, as it is a micro-nutrient. You also need all 17 nutrients so a comprehensive fertilizer would be better, but that can increase algae all back to the balance so I would leave it for the present.
Due to long illness & inattention to everything except water quality, my 40 gallon is full of algae. (It's too close to a window.) Result? Blue gouramis came into spawning condition, made babies & are working on their second spawn. Some of the first spawn have survived thus far & are growing w/no added infusoria. My point is that this type of algae (exactly like your photos & what you described) hasn't hurt my gouramis or the 4-5 inch clowns (plus 2 green corydoras). The cories, oddly enough, are in full breeding coloration, too. I can't advise as to removing algae, but if your algae is the same as mine & your water quality is good, it will help rather than hurt the fish. In a show tank, I'd certainly try to get rid of it. Pristine tanks are beautiful, but I started letting algae grow on my tanks' back & sides & plastic plants & driftwood years ago; my fish do so much better in a low-stress environment. Good luck. I look forward to reading about getting rid of unwanted algae now that I'm all excited about my hobby again & want to have one tank that's pristine.