I plan on getting two 150Watt heaters.. is it safe to get the Tetra Whisper Heaters from Wal-Mart? Or should I get more Penn Plax? I have a Penn Plax 150 in my 20Gal right now.. runs great.. but was expensive from the LPS.. only options are those 2.. unless I order online.. and I kind'a need a heater now.. What do you think?