Changed Substrate - Nitrate down to 0
 

Aug 24 - 0 Ammonia, 0 Nitrite, 20 Nitrate (using API Master Test)

Aug 31 - 0 Ammonia, 0 Nitrite, 20 Nitrate

Sep 6 - 0 Ammonia, 0 Nitrite, 20 Nitrate

Sep 9 - 0 Ammonia, 0 Nitrite

Sep 11 - I changed the substrate from gravel to sand. I also did a 50% water change after stirring all of the gravel, the water was dirty. (with 0.8 ml Prime and did not touch the filter)

Sep 12 - 0 Ammonia, 0 Nitrite, 0 Nitrate

Sep 13 - 0 Ammonia, 0 Nitrite

Sep 13 - Did another 50% water change because now I'm able to see the fish waste a lot clearer on sand. (with 0.8 ml Prime and did not touch the filter)

Sep 14 - 0 Ammonia, 0 Nitrite, 0 Nitrate

Should I leave the tank alone until I get some sort of reading?

10 Gallons
2 Sailfin Molly (1.25")
1 Albino Bristlenose Pleco (1")
1 Golden Mystery Snail

Do you have live plants in this tank? When you changed the substrate did you make sure to keep your filter media wet during the change over and any other decorations in the tank such as driftwood, rocks and such. If you kept the filter media wet and the other decor than you will have bacteria that will have stayed alive on that. Also if you have enough live plants in the tank this will help as the bacteria reestablishes itself in the tank. I would continue to keep an eye on the water parameters, but as long as your ammonia and nitrites stay at 0 I would only do your weekly water changes. If ammonia or nitrites go above 0 go ahead and do a water change.

Thank you for the quick response! :)

I have 3 Argentine Swords doing ok in this tank.

I put all the decoration, drift wood, plants into the same bucket that I was emptying the water from the tank so that it would stay wet with the tank water.

Not 100% sure on how wet the filter stayed, I cut the power first, so that the water wouldn't just empty out while I was draining the tank.

I'll monitor the ammonia and nitrite daily. Hopefully the bacteria will reestablish itself.

Thanks again!

You should be fine, looking at the stocking on both of your tanks, you are not heavily stocked on either one for fish. Enough of the bacteria should have survived on your plants and decorations to be able to reestablish itself, and I would also imagine as long as there was water in the filter, there would also be bacteria on the filter media. Taking both of these facts into account I would say that you should not have any problems ;-)