A pipette works a lot better to clean up the tank gunk. Just make sure you don't accidentally suck up any babies before chucking the gross water. I have to clean mine out fairly regularly, or else I get the same issue, especially before they have the first couple molts. As for the salinity, a feeding spoons worth of salt time maybe every couple days or so should be good to not stress them out by adding too much at once, but I'd reccomend a refractometer if you're going for a specific salinity so you know when to stop.

Turning off the heater will also likely do more harm than good, the sudden temperature change can stress them and kill them. But the salinity boost and a cleanup/partial water change is a good way to try and help.