Hi there

I have two external Fluval filters & with mine, I use carbon, a Nitra-Zorb pillow & the weird looking ceramic things that the water filters thu & of course they both have sponges too.
You don't have to add the carbon or Nitra-Zorb, but I prefer to use them as they filter out the nasties.
I don't know if I have been able to help you tho, MrsM.
<p>MOBY TANK:<br />1 South American Puffer, 2 Jullii Corys, 2 Neons.<br /><br />FLUVAL UNO 500:<br />6 Ruddy Nose Tetra, 5 Cardinals, 1 Bristle Nosed Plec, 1 Candy Striped Loach, 1 Dwarf Frog, 6 Amino Shrimps.<br /><br />JUWEL RIO 125:<br />2 Plattys, 1 Silver Shark, 2 Bristle Nosed Plecs, 9 Guppys, 6 Amino Shrimps.</p>