I know that as you are a new aquarist, all this must be rather overwhelming. Stay with us though, we will get you through it.
First general advice: do not add any additives, treatments, medications to the tank water without very good cause. Diagnosing issues like disease is not at all easy for even the experienced aquarist, and as these substances can cause stress they are worse than doing nothing. A good conditioner to deal with chlorine, and chloramine if that is in your tap water too. Most conditioners will do this, the API Tap Water Conditioner is your best one here, but the one you use now is likely OK until it runs out.
On this, what is the "stress reliever?" If this is something like Stress Coat, forget it. None of these things are necessary, and they do affect fish negatively.
Planaria are small white flat worms that live in the substrate, and may be a sign you are feeding too much. There are other internal worms. Metronidazole is not going to do anything for planaria.
Water changes need to be regular, no less than once each week, and should be substantial, at least half the tank volume. In any aquarium. Do not add salt, just the conditioner. As for mixing tap water and distilled/RO water, this may not be advantageous. We need to know the parameterrs of your tap water on its own, this means the GH (general hardness), pH and KH (Alkalinity). You might be able to get thi data from the web site of your water authority. It is important to know these parameters before getting any fish. You did mention the water hardness as 130, which I assume is in mg/l or ppm, but please confirm. There are different units to measure GH and KH, we need to know which.
Do a good clean into the substrate (gravel) with the water changer each time to change water. And keep the filter clean, rinse all the organic matter (the brown gunk) out each time.
As for food, do not overfeed. Bloodworms are not nutritious, I would use a couple of good foods. Fluval Bug Bites are ideal for all omnivorous and carnivorous fish. The herbivorous fish that eat plant matter or algae need a "green" food.