My guess is that cleaning too much caused a mini cycle.
I'd get the sunlight blocked and let the tank settle for a while without messing with it much.
Oh no- the cleaning was in response to the algae, not what set it off, it had been a while since I had done a good scrub of the tank walls and the BGA had been growing over a period of a couple weeks prior. The first cleaning was to get sheets of it off my side wall (not actually getting any sun) and the back wall (got a little sun), and then the second cleaning a few days later was just getting a bit that I had missed. I haven't actually seen any new growth of it yet but I know it's still in there. I had been a little concerned about whether the double cleaning would interrupt the cycle so I had actually tested it the day after the second cleaning and it was Ammonia 0 / Nitrite 0 / Nitrate 15. That was two days ago now so I'll check again tonight.
But yes, just about to get the curtains back up as we speak.
I'm battling blue/green algae in my high tech 55 gallon. Cyanobacteria does best in low oxygen and low nitrate environments and is a sign there is something wrong with your biological filtration. You need fish to keep the cycle going. The good bacteria in your filter need the waste from your fish. Biological filtration also needs oxygen. Cyanobacteria is capable of producing it's own nitrogen. Killing it can cause it to release toxins into the water that it has stored up. Best to find the cause then bomb the tank with chemicals.
Try running a small airstone at a low rate and add some hardy fish. Or if you're using a hang on back filter try not to over fill the tank so the filter flow can break the surface of the water up. Oxygen will enter the aquarium with surface agitation. There is a relationship between your bio filter fish and plants. Once you figure out how to balance the 3 the BGA should clear up on it's own. Make sure you have enough bio media. I usually cram my filters with as much as it can fit. Most plants grow so slow in a low tech they need very little nutrients.
A 5 gallon can be pretty difficult to manage because of it's small size. Planted tanks don't take well to fast changes in water chemistry and temps.
Thanks for the ideas! I'll pull the baffle off the filter and lower the water level. I had a betta before and they like slower current and can breathe air from the surface so I didn't have an air stone, but that's something to think about. I was planning on getting another betta in there as I can't really think of too many other fish that could deal with being in such a small tank. I guess maybe 3 or 4 guppies but I'd be too worried about them breeding.
The tank has been cycled for about a year with weekly water tests, unless there was a mini-cycle that somehow lined up just perfectly in between water tests (stranger things have happened so I can't discount the idea lol). I do have a HOB filter that's literally stuffed to the top with several kinds of sponge and a bag of biobeads, so I didn't think that was the problem, but I'm looking at all possibilities.
I have a nerite snail in there right now who is a poop machine and the nitrate levels have been between 10 and 15 even after the betta died so I was hoping to have a tiny bit more time before adding a fish since I wanted to do some hardscape. Do you think it would be better to just go ahead and get the fish in there and finish scaping after?