Dear Taner,
thank you for your kind words.
I think keeping Piranha in planted tanks are a great challenge. If you want avoid big water quality problem (like Phosphat/Nitrat) very very important is a water change frequently, a minimum 30-40% every week. Especially when you feed the Piranhas strong.
For plants covering the wood Microsorum or Anbuia will be a good choice. Maybe also some moss sorts.
For covering the floor: Echinodorus tennellus, Cryptocoryne parva, Cryptocoryne are a good choice for Piranha tanks.
Please take care because of the water temperatur: 26 C and not more! If the temperatur getting over 28 C the Piranhas start to eat the plant to prepare spawning.
Best regards, Oliver Knott