Ah, blue algae (actually blue-green). I think that may be a key to the dark blue fish we catch in the wild. The sp. Pebas and urteagi are examples of two that were both caught in cow pastures that were rich in blue green algae. Once in our aquariums they tend to fade. Strangely at least to me spirolina does not help the coloration.
You made my day. I have two tanks with serious blue-green algae but I've been hesitant to give them a thorough overhaul as there are breeding apistos in them. Some of the biotope footage also shows plants heavily coated in detritus and algae with happy fish swimming all around. I bet the algae mats are a smorgasbord of food for small fish and fry.
I'm a bit of of a <"periphyton, biofilm or aufwuchs"> fan as well for fry.Here is a pic of some sword plant leafs covered in algae with some of the Apistogramma ortegai fry that got their first outing yesterday.
There's a LOT of different blue-green algae (cyanobacteria)! The slimy dark mat-forming ones that are common in aquaria (Oscillatoria, Phormidium, etc) are not bad about releasing toxins (although i've never found any fish or invert that eats them). Some other cyano's are far more toxic, but don't often bloom in aquaria.
The blue neon goby, Stiphodon atropurpureus, is one that is said to eat, or even prefer, blue green algae. One of our fish stores had them a while ago and I was severely tempted as they are beautiful and with interesting substrate-perching and burying behaviour. Their life history with migration to sea for larvae and return back up the river for subadults (like Amano shrimp) is also crazy. But I skipped because they are Asian, no breeding, and bottom-associated so likely to get into a spat with breeding apistos. If they were selling them now I would get a couple just to test the veracity of them eating blue-green algae.