Some tap water may contain sufficient nitrates and phosphates (NP) to sustain healthy growth. They can also be provided from fish waste and food. Potassium (K) is not normally present in either though, in decent quantity.
The plants' nutrient requirement depends on your set up. I you have lots of light, CO2 injection etc. then plant growth will use up nutrients quicker, so you'll need to replace them, even in tap water with high levels of NP. Even in lower lighting, but heavy planting, NPK can be added to improve plant growth. NP alone doesn't cause algae in well-planted tanks.
These days folk tend to run more light, more CO2, more demanding plants, so the need for regular fertilisation is crucial to prevent nutrient deficiencies.
One of the most effective, simple, safe and cost effective ways to achieve this is by using the Estimative Index.
Basically the aquarium is "overdosed" with nutrients, including nitrates and phosphates. Nutrients are dosed daily, micros one day, macros the next, alternating. 50% water change every week resets nutrient levels. Simple!
For more info check out JamesC's excellent pinned thread here -
http
/www.fishforums.net/index.php?showtopic=104737
You can use the Seachem products but it is much better value to buy the dry chemicals and "roll your own".
Your tank may well benefit from NPK addition.
HTH,
a plant freak...
