Originally Posted by Wasserpest
Most of the commercially available aquarium plant nutrients do not contain (at least in noticeable amounts) Nitrates or Phosphates. Yet both are required for healthy plant growth.
Adding nutrients per se does not lead to algae, however, if your plants are already dying, or some other nutrient is missing, it might seem like it. Google "Liebigs law of the minimum" for a good visualization.
Plants need ALL nutrients in small amounts. Adding something every 4 weeks seems not a very good approach. Most of us dose DAILY.
So... my recommendation would be to get some KNO3 and KH2PO4 (if available where you live), read up on dosing, and then start in small amounts, and monitor your tank over longer periods. Yes, initially you might get an increase in algae, as your plants are not in their best shape to process the nutrients and grow. Perhaps adding some healthy plant mass along with the nutrients would be a good idea too.
would you please take a look to my photo album no:3 to evaluate my situation. by the way today i bought "Test 6 in 1" to test my water, i think the kh & gh are very hi. when i make the test i will post the results to you.