I would approach the problem this way:
Step 1: Remove nitrates from the tap water, or use a water that has none. This is a small tank, it will not blow the budget to buy RO or distilled water for it. Use tap water ONLY if it is proven that the plants are capable of handling the nitrate load.
Step 2: Redo the tank, removing the substrate (and its overload of Osmocote).
By removing two of the three sources of NO3 you are left with the protein in the food as the main source, and every fish/shrimp keeper deals with that. A good assortment of plants ought to be able to cope with that much nitrogen.
Comment to two of the people who posted on page one:
Excess NO3 is not a lack of or incomplete nitrogen cycle.
If there is no other source of NO3, then high NO3 is a sign that the cycle IS complete.
Seeing ammonia or nitrite (NO2) are signs that the nitrogen cycle is not complete.
In this specific case the OP said the tap water tested positive for NO3, and later posted about Osmocote in the substrate.