If the water cycle shuts down to such a degree that the desalinated water is not making it back into the oceans, we have planetary-scale problems far more worrisome than a slightly elevated ocean salinity.

If you had an absolutely huge number of these in a small area, I'm sure you could probably create a localized disturbance in the salinity. But 13k gallons is a pretty trivial amount. That's a 50 meter cube of water per day... in the ocean.

With the rising sea levels due to glacial melt the saltiness of the water is going down, so really this is just doing the fish a solid! /s

Assuming:
Current ocean salinity = 35 kg / m3. Current ocean volume = 1.4 * 1018 m3. Current human fresh water usage = 4 * 109 m**3.

(35 * 1.4 * 1018) / (1.4 * 1018 - 4 * 10**9).

= 35.0000001 kg / m**3. = New ocean salinity.

I think they'll be okay.

I get what you're saying, but it's not like we're just taking that water and blasting it off into space. It's going to be used and then make it's way back into the ocean eventually. Either through evaporation and precipitation, or directly through rivers that feed into the ocean.

Of course, if there hasn't been already, there should definitely be some extensive studies into whether there are any blockades as far as that water returning to the oceans in a reasonable amount of time.