To the test numbers...
You should be able to confirm (or not) the GH and KH by checking your municipal water authority's website. GH and KH are not likely to change much in an aquarium, unless they are specifically be targeted (we needn't go into that now).
The pH is not necessarily low. pH is determined by several related factors, including the GH and KH. If you find the water authority's site, see if pH is listed. This will tell us how much it may be changing in the aquarium. It is natural for the decomposition of organics to produce CO2 which forms carbonic acid and thus the pH lowers; this is a simplistic short explanation, but my point is that it is natural. The GH/KH help to maintain the pH and prevent it from lowering too fast or too far, but the extent to which this occurs varies according to the source water and the tank's biology (fish load, food, water changes all factor into this).
By the way, the relatively low GH and pH may be the cause of the shrimp disappearing; harder water is needed for some species, as they need calcium for their exoskeletons. I would not get any more shrimp, at least not until we have everything else sorted out. And if my surmise is correct on the GH/KH and pH, soft water fish would be better here, like the neons.
NO3 (= nitrate) is high, but we are dealing with test strips which can be a bit off sometimes. See if nitrate is mentioned on the website, dor test the tap water on its own if you still have nitrate test strips, for comparison. Nitrate can occur in the tap water, which is one issue, or if not then it is coming from within the tank, and this we can deal with more easily. Larger water changes (50% of the tank volume, once a week is what I tend to recommend generally) should handle this, along with not overstocking or overfeeding. And doing a good vacuum of the substrate during the water change is important to get much of the organics out of the substrate. And keeping the filter media rinsed.
The algae wafers, do you mean sinking food wafers for the plecs? These you should be able to suck out with the substrate vacuum during the water change. If they are still there after a day, it likely means too much food is entering the tank.
Byron.