There exsits quite a lot of confusions about filtration. Filtration is closely to flow rate. However, most people don't realized its importance. For example: 80% of people having aquarium in their home, have insufficient filtration system installed on their tank. To more extent, when you choose lab filtration products like syringe filters, cartridge filters for filtration, you must pay much attention to its flow rate. So you see how important of a qualified products when you do filtrations!

What is a flow rate? Flow rate is how many times the water pass through the filter in a hour. Example: you have a 20 gallon tank, you use a HOB filter that pump about 100 gallon an hour. This means you are turning over 50% of your total water volume an hour. Is this enough? Unfortunately, no! At 50% you are only running 100 gallon an hour, the optimal flow rate for any aquarium is 10 times an hour - fresh, salt, pond all the same.

A 50% turn over ratio is too slow, not everything get taken out by the filter. Too fast of flow rate, not enough resident time for the filter and bacteria to process the waste. What you need is an optimal flow rate and that number is 10.

Everything you ever read in the past said differently, 5 times for fresh water, 10-15 for salt water some even suggest 20 times the flow rate for salt water. This is what the majority of people are running right now, and the same people are having some kind of problems day in and day out. How many times have you hear people say:

I can't keep more then 4 or 5 fish in my tank?

I just can't keep anything alive for long?

My nitrate or ammonia are high. Salt water are too difficult to do.

I like to keep discus but they are too hard to keep.

If you take a close look of their problem you will soon discovered the root cause of their problem can usually be trace back to one of the following: insufficient filtration: either too fast or too slow, too much fish or livestock and over feeding.

How much filtration and how many filter do I really need?

The filtration you need depend on your flow rate and that in turn depend on the size of your aquarium. 

Now that you've establish the flow rate of your newly purchased filter, can you honestly said this filter can adequately filter a 100 gallon tank? Even if the filter is pumping at 350 GPH you'll still need minimum 3 of the same filters to adequately filter a 100 gallon tank. Somebody will point out 350x3 = 1050, is too fast. No is not! Remember all filter will clog sooner or later, as the filter begin to clog, the water flow will decrease so as your flow rate. You always want to go higher then the required flow rate not under. If is too fast you can turn it down, but if you go under, there is no way to dial it up other than change pump or filter.