Help My Fire Belly Toads are dying

[PAFishMan]

Ok, I just did a water change and before I was told I took that wall out in the back lol. I may put it back if I can get things stabilized first. Here is the thing, I took out all the water ran the gravel through a strainer not a whole lot but I thought enough. The pet store told me to use tap water with a reversal osmosis. I did that we have well water with a reversal osmosis. We are in a farming community so I replaced half with distilled water and the other half with tap. I took some pictures of what it looks like now. See the pictures below. I did test the water after setting everything back up and I was shocked. The Ph is 7.2 it was 6.4, the High Ph is 7.4 it was off the chart before so now that is normal. The Ammonia is 1.0 ppm it was 8.0 so that is a lot better, The nitrite is 2.0 ppm it was 0.25 like I said we are in a farming community and to my surprise the Nitrate is exactly the same 160 ppm?? What happened? Did that much get put back in from the gravel. I cleaned the little filter in the pump even. There was some dead crickets behind the wall but I cleaned that out and washed out the tank in a sink. I ran water into the tank and dumped it a few times until the water was clear and dumped that out to. I had all the gravel in a five gallon bucket and put some into a strainer and washed it a little then dumped it into the tank and kept doing that until I had the gravel back. I guess I will check it again in a couple of days, that is if I still have these two frogs. I am going to put some Prime in now as well. If anyone can let me know here how I can get this cleared up faster let me know. Thanks