White patches on ACF's throat

[Gemma]

A couple of days ago, I noticed some white patches on one of my African clawed frogs' throats. I then examined the other 2 frogs in the tank and noticed that they have the same thing, but to a lesser extent (I probably wouldn't have noticed it on these frogs had I not been looking for it). The white patches are in the same position on the throats of all 3 frogs and they have no patches elsewhere on their bodies and no other signs of illness. As far as I can tell, the patches are smooth like the rest of their skin. They are behaving completely normally and eating well.

Background info, in case it's needed:
Tank is 180 litres. It has 3 female ACFs in it and no other animals. Sand substrate. Decor is driftwood and live plants. I keep the water at 21 degrees(C). I do 25% water changes every week and condition the water with seachem prime. I use 2 filters: fluval 306 external, and juwel eccoflow 600 internal. Last readings: NH3 - 0ppm, N02 - 0ppm, N03 - 20ppm.
I've had the frogs for 4 years, since they were froglets, and I haven't added anything new to the tank for a long time. I did a water change 2 days before I noticed the white spots, and obviously temperature matched and conditioned the water.

I'm away from home at the moment and not back for another 3 days, so I'm worried something may happen to the frogs while I'm not there. Before I left, they were eating earthworms and there was nothing odd about their behaviour at all.

Does anyone know what the white patches could be and if I should be heading back early to check on them?

The frog that's got it the worst:


It actually doesn't look as bad as this in the flesh, but they're the only pictures I have where you can see it.

The other frogs (blurry picture, I know, but you can kind of see):

[Gemma]

I don't know how to edit/delete this thread but I've just been to check on them and they're fine

[tgampper]

Could be a form of fungus, such as water mold (genus Saprolegnia). Look for cottony white tufts in the water. This mold is only loosely attached to the frog's skin. It is generally not a serious disease unless it is accompanied y a bacterial or viral infection. This species has a built in medicine chest in their skin and often they can cure most diseases. It is good that they have cleared, but you still need to be vigilant.

[Gemma]

Thanks, Terry. I didn't notice any cottony tufts on the frogs or in the water at the time but I'll look out for them and keep an eye on the frogs in case it happens again. Thankfully, the white patches had disappeared by the time I returned to check on them a few days later and they haven't returned. Do you know what causes the fungal infections, if it was that?