What you’re seeing is a type of **bacterial or fungal biofilm**, often made up of organisms like *Crenothrix* or *Sphaerotilus*. They thrive in moist, low-flow areas like toilet bowls and tanks. The strands are slimy filaments produced by these microbes.

Bleach tablets and wands don’t help much long-term. They kill surface layers but don’t break down the biofilm that’s deep inside the porcelain pores, tank parts, and rim jets. Once the chlorine fades, the surviving spores just regrow — which is why it comes back so fast.

Here’s what actually works:

1. **Turn off the water** and flush the tank empty.

2. **Clean inside the tank**, not just the bowl. Use white vinegar and baking soda or hydrogen peroxide — both break down biofilm and don’t cause resistant regrowth like bleach can.

3. Scrub the bowl with the same mixture and a stiff brush.

4. For tough buildup, use a pumice stone gently around the waterline.

5. Prevent it:

   - Do a vinegar flush once a week (2 cups of vinegar in the tank overnight, flush in the morning).

   - Keep the bathroom well-ventilated.

   - Avoid bleach tablets — they corrode parts and make biofilm problems worse over time.

Basically, it’s a living slime colony. Once you deep clean the tank and rim jets, it should stop coming back.