So, Sunday I shook off the dust from my trusty old wallet and lavished out on an external filter. EHeim Professional Pro II. Now all the articles and advise I have read here and elsewhere kind of says that you should turn over the volumn of the tank about 10 x's per hour. I only have a 90cms tank and depending on which calculator you use can be anything from 105 litre to 153 litres( hummmmm ). Quick ask about, and an educated thumb suck and I decided about 100 - 110 litres. Therefore 10 x 100 = 1000 litres per hour. Bought a 950 l/h machine. Must say the quality and detail of the product was great, even a small sachet of vaseline for the O'Rings, includes all filter material, tubeing, suckers, clamps etc etc. One stop shop. Without much effort everything was in place and the grand switch on was enacted. Well, faceplant of all fish against the far glass panel. Fortunately, there is an adjustment to control the flow. Got it all stabelized and settled. My question is - is 10 x per hour really necessary or did I just mis-interperate ( hope theres a spell check here somewhere ) or was it overkill for my size tank ? I must say that watching the danios etc speed surfing from one end of the tank to the other is really exciting, although I think the harlequins are suffering a bit with exhaustion ( wheres that spell check ). Apart from the Harlequins all the other fish seem to have settled down. The water is clearer than ever and to see the plants swaying with the flow is very satisfying.