I have an early Decent Espresso machine. I originally pre-ordered this from ZPM Espresso, who failed and where then rescued by Decent.
I really like the machine, but it has one significant flaw for my use case - the water reservoir is a ceramic container which sites under the machine, and, if I don't use it regularly, grows a large amount of algae or mold. The environment seems almost designed for algae growth - the machine keeps it nice and warm, it's dark, and there is a large amount of surface area. In theory I could just empty the reservoir after every use and refill it before using it, but I often, um, don't.
A few years ago I mounted a set of UV-C LEDs to the bottom of the machine, facing the water reservoir, and this helped immensely. I meant to document this modification, but never got around to it.
Recently the cheap LED driver, or the LEDs themselves stopped working, and so I replaced it with something better... and this time I'm actually documenting this simple modification.
The UV-C LED.
I used a UV-C LED source from AliExpress, designed for water purification. The one that I got was "DC12-24V UVC 270-280NM Ultraviolet Water Purifier Sterilization Pet Water Dispenser Humidifier LED Disinfection Module" and was less then $5.00USD (AliExpress items and descriptions change quickly - if this link doesn't work there are likely many more). I do have a spectrometer that will do UV, but its bottoms out at ~330nm, so I couldn't really tell if it is actually UV-C, but based on how well it works, and also how it look, I think that it has at least some UV-C wavelenghth.
To install the LED:
Remove the outer shell - it is held on with some relatively small torx screws, then remove the bracket which holds up the solenoids, etc.
Use a step-drill to make (or enlarge) a hole so that the glass / quartz cover can fit through. Because of the clearance between the bottom of the machine and the top of the water reservoir only a small part of the unit sticks through.
I considered drilling two additional holes to mount the mounting tabs, but discovered that simply tucking one of the tabs under the sheet metal holds it well enough. I added a bit of silicone to provide additional structure, but this probably isn't needed.
There is a small, black (at least one mine) unconnected 24V socket. I didn't have a matching connector handy, so I simply cut off the connector and stuck pins in the socket.
Then test to make sure that the LED works, reassemble, and you are done.
This was a quick and simple modification, but so far the water has remained algae / mold free since installing this.