When I cycled my tank I had the same exact issue and I did the same thing with adding the shrimp. Your LR
is probably cured already and helping to keep the numbers from rising too high. Wait and see if your nitrites start to go up after the increase of ammonia. Or if your ammonia bottoms out to 0. Since you are getting 20ppm of nitrate that does mean there is a little cycle already. However, you need to wait for that ammonia to go down to 0. Is the "master saltwater test kit" stirps? If so stop using those and get liquid/test tube test kits. My advice would be do not do a pwc
yet. Wait until the ammonia/nitrite is 0. I'm sure some of the others on here will not agree with the next part I am about to say but: like I mentioned I had a similar issue becaues my LR
was very cured. I couldn't get the ammonia down lower than .25 and I never ever registered nitrite. After about 6-weeks of this issue, I bought some of that nitrifying bacteria in a bottle "SuperBac" I think it was called. I dumped that in my tank and it dropped my ammonia to 0. I then slowly started stocking and have not registered ammonia again.
But that stuff in the bottle is definitely not a cure all for cycle. It didnt work out so good in my 10gallon QT.