Yeasts.

So, if Saprolegnia and kin are water molds rather than fungi , and the "slime molds" aren't really molds, then the true fungi most commonly found in water environments, are microscopic ascomycetes and yeasts. (There are also a few of the "conjugating" fungi, or Zygomycetes, which a fishkeeper can ignore without seriously distorting the general picture.)

Yeasts make a living as nutrient recyclers. They don't form a unified network of mycelium, like most fungi. Instead, they remain as individual cells and small groups, constantly budding off new cells and pinching off from the parent cluster. Candida is a notorious yeast that normally lives harmlessly in our own damp areas but can produce the rash called "thrush." But in freshwater, yeasts more typically use non-living substrates. Normally they secrete enzymes that break down carbohydrates to sugar, and then they metabolize the sugar. On the whole, yeasts thrive with warmth and moisture wherever they can glean some oxygen, but when oxygen supplies start to run low, yeasts are also capable of a fermenting metabolism. Fermentation may be less efficient in generating energy, but it doesn't require oxygen.

There are some yeasts scattered everywhere in the biofilm. Normally, they play only a small role in the aquarium, I understand, yet yeasts manufacture B-vitamins, which vertebrates can't make for themselves, and those essential vitamins get passed up through the food web. I'm often made aware of this role when I feed microworms to fry and catch a whiff of that yeasty odor.

When you think of yeasts, you might correctly include among the aquarium's menagerie of fungi the yeast working to provide carbon dioxide in that DIY CO₂ soda bottle, which may be providing carbon dioxide for your planted tank.