I am a bit puzzled, having read this thread through. I do not understand ammonia at 8 ppm with all these plants. Bacterial supplements can do no harm, but they should not be necessary with plants.
While it is true that changing the filter media would have removed the nitrifying bacteria in the filter, there are bacteria elsewhere in the tank, not to mention the live plants using ammonia/ammonium.
Maybe we are not clear...just how many fish are in the 10g tank? Assuming this is the problem tank.
Both tanks are problem tanks, neither one has ammonia levels below 8, and actually they are probably higher than that, it's off the charts.
First ten gallon tank has 6 adult guppies, roughly 15 baby guppies of varying sizes, most are still very little.
Second ten gallon tank has 6 adult guppies, 2 baby guppies of very small size. This tank used to have 10 adults, but 2 died and I moved 2 to the other tank.
For the first month I didn't test the water, as the fish all seemed to be fine and the store told me I didn't need to---big mistake, I know. During the first month I didn't really suck out the gravel, I didn't know how, I just ran the cleaner over the top of the gravel. I was probably feeding too much at first too.
The first time I checked for ammonia was after I changed out the filter media and had finally decided I needed to buy dropper test kits. It showed .5 at first, then after a couple of days and a BIG gravel cleaning where I pushed all the gravel around like a novice and made stuff come up into the water, it ran up to 4.0, and then within a couple of days, over 8. And this is doing 30-60% gravel changes each day since the ammonia spike. It's like the bacteria product I was adding was made up of ammonia or something...API Quick Start.
Tonight I just changed out 50-60% of the water in the tank #1 with most of the babies. I carefully cleaned the gravel, after removing the plants and ornaments, most of which are potted, now that the LFS finally showed me how to do it correctly, but not all that much stuff was in the gravel at this point. I rinsed the media in the tank water. Then I retested the ammonia, and it was still 8.0! It must have been 16.0 before that, but I am still at a loss. This test works correctly, and tests 0 for our tap water.
As for the types of plants, three potted plants per tank--two swords and one arrowhead, or that's what I was told. Also two bunch plants per tank, not sure of the name, they are "fluffy," spread out in each tank, and one other "bunch" plant. No floating plants. I'm sorry I am not knowing the names of these plants yet.
I haven't changed Tank #2 today yet, the one with the six adult guppies. Waiting for advice....
Thanks so much, I know this seems to be impossible, but I am looking at the test tube and shaking my head.
Also, a handful of tiny snails have grown in each tank, having hitched a ride in on a plant.