Nice speed controller Hansfragger!
To bad that the MD20R runs on AC, it just makes everything much more expensive! A variable AC transformers are both big and expensive, and they might even emit a more annyoing sound than your pump!
I'm
not an electrician but maybe you could use something less powerfull like a
dimmer? Those variable AC transformers can handle loads of current, your pump draws maybe 0.5-1A? The easiest and cheapest solution would be to just pack lots of sound isolation around it. (Or do like me and buy some earplugs!
)
Btw. the miniNG has two channels and in analogue mode it can handle 20W on one channel or 25W if you combine both channels. In PWM mode you can use 50W per channel or 100W combined.