I'm sorry this is late (I'm in school).
Your heater needs to be changed out for something larger. heaters are typically 5 watts per gallon in order to do any good at all (so a 50 watt heater for your 10 gallon). You could buy an aqueon 50 watt heater from petsmart for about 25$. It's honestly worth the investment, because with heaters, you get what you pay for.
One way would be to, mix up a batch of 1 and a half tsp of aq salt per gallon, and then do a water change and replace with the new saltier water. In a few hours, mix up a new batch of water (2 tsps of aq salt per gallon) and do another 100% water change.
Another way to fix it would be if you know exactly how much water is in the tubberware. Figure out how much of a gallon it would be (as an example, let's just say it is 1/4th a gallon), and then dose the tank with 1/4th a tsp of aq salt -that would equate to 1 tsp of salt per gallon. Later, remove a tiny bit of the water from the tubberware into a small cup. Mix an additional 1/8th a tsp into the water (be sure to dissolve properly), and then add it back in. That would bring the salinity up to 1 and 1/2 tsp of salt per gallon. Later, remove some water into the same cup and mix an additional 1/8th a tsp of salt into it and then pour back in. Now your fish would be in 2 tsps of salt per gallon. (*I really hope this makes sense. If not, just ask and I'll try to rephrase*)