WCMM aren't true cold water fish, they come from subtropical areas. 16c is a bit on the low side, if its going lower than that at night it won't be helping them.

Can you use some tank decor to wedge/support the heater so its off the tank floor and not touching the glass? It would also benefit from being in an area with good flow. If you do get the heater running make sure not to warm the water too fast.

As already stated, WCMM are temperate and do need to have stable temps; preferably around 18-20C so a heater is essential if it's dropping below that. You can get clips/suckers on eBay.

Water changes are usually at least 25% weekly, more if nitrates are accumulating rapildly.

This type of fungus often occurs due to stress induced by poor water quality and low or fluctuating temperatures. Best plan is to get the heater running and gradually bring the temp up to about 20C, increase water changes to 25% weekly and treat with a good anti-fungal/anti-bacterial. eSHa 2000 is very good and can also be had on eBay.

Bad news it spread, and it just died. I'd just added phenoxyethanol, as they had no Esha 2000 :(
Could it affect the fry?

AFAIK it's more usual to treat the affected fish in a bath with this rather than the whole tank (unless most tank occupants are affected).

I'm not sure how it will affect the fry, but as it's also used as a sedative/anaesthetic for fish, and in increased doasage for euthanasia, it's effects on very young fish may be detrimental.

I'm shocked, didn't think it happened so fast :(
The staff member told me it had to go in. I stupidly put off putting it in because I meant to ask you all. The info leaflet just had a dosage, not how to use it.
Really annoyed with myself. The fish looked doped a bit.
It said to repeat until symptoms disappeared, should I ?

I'm afraid I'm sketchy on the use of this as have never used it myself. If the fish are already looking a bit doped up I think I'd be inclined to hold off unless there are a lot of fish badly affected. How many are showing symptoms?

It sounds like columnaris which can be very nasty, so be prepared....it can take hold and progress very rapidly

If it's already progressing through the tank you're a bit between the Devil and the deep blue sea on this....if it's that bad, you should probably risk the treatment as some may come through with that, whereas untreated you may lose them all. So sorry....

Only that one is affected. I thought the fry would be more likely to go first, and tothink I was starting to worry less then this happens. The staff member told me to change water less to build their immunity so my family still think I'm doing wrong insisting on water changes.
None of the rest are affected but I'm terrified as that one went down so fast. Was so busy with graduation and all. Now feel dumb as Dad bought the car today and I insisted on getting to shop to get a treatment, but also went and got graduation clothes. Maybe it'd still be ok if I had immediately treated it.
Don't feel sorry for me, feel sorry for the poor fish for having been landed with such a ditz. I'm learning the hard way to follow the rules to the letter.

Ps Esha contains copper which WCMM are sensitive to, so would the phenoxyethanol be a better choice? The staff member said salts wouldn't really help and were more for prophylaxis. Then again the rest of his advice wasn't great. Not that I can blame him as it's my fault this mess happened in the first place.

Salt wouldn't really be enough given how severe this seems to be - it's better as a preventative where there are wounds from fin-nipping and so on. Once fungus has taken hold it's unlikely to be effective.

However, as the affected fish died and no others are showing symptoms I think for now good clean water and adding a heater may prevent the others from succumbing. Hold off on treatment unless you see symptoms in the other fish - but do be vigilant.

eSHa products are claimed by the manufacturer to be safe for all tropical fish as long as you are careful to dose correctly. I've personally used their products with sensitive species' in the past and seen no issues from the meds and very positive responses in terms of dealing with the conditions being treated. Do make sure if you decide to use the eSHa meds that you do a water change and run carbon to remove the other treatment first.

What size is your tank?

If its columnaris, then perhaps you should get ready to use antibiotics if other fish start showing symptoms.