The easiest way to choose a filter is by understanding turnover. This is how often the water is pulled through the filter. Ignore the manufacturer's recommendations as these are usually based on optimistic conditions, a bit like how motor manufacturers quote the mileage of their cars, or cereal producers how many portions are in each box.
For a lightly stocked tank (i.e., one with small fish like neons and guppies) you need at least 4 times the volume of the tank in turnover per hour. So a 150 litre tank will need a filter rated at 4 x 150 = 600 litres per hour.
For a moderately stocked tank (i.e., one with medium sized or somewhat messy fish, like gouramis and angelfish) you need a turnover of 6 times the volume of the tank, in your case 6 x 150 = 900 litres per hour.
For a heavily stocked tank (e.g., goldfish, cichlids and plecs) or one with fish that need strong water currents (e.g., loaches, hillstream fish, Tanganyikans), then you need a turnover of 8-10 times the volume of the tank, i.e., 1200 - 1500 litres per hour.
With few exceptions, you can't go wrong using more filtration that you might imagine. Very few fish object to strong currents, and most prefer much faster water than we give them in captivity. Stronger currents mean more exercise for them, so they behave as if they're in a much larger tank. They have to work harder getting food, and this makes life more interesting for them.
While everyone has their biases, it's hard to go wrong with a decent external canister filter or a properly maintained undergravel filter, if you can live with the limitations of the latter design in terms of plants and rockwork. Both provide optimal value for money. Internal canister filters can work well, and are very easy to maintain, but are expensive for what they do in terms of money spent versus turnover and filter capacity. Hang-on-the-back filters are easy to maintain but are not particularly efficient at creating uniform water flow throughout the tank, and I dislike especially those models that lock you into using filter media cartridges that serve no real value, like carbon. Air-powered filters are worthwhile only in relatively small tanks, but in those situations they excel, e.g., with fish fry.
There is no reason you cannot use two or more filters on one tank, and in fact combining two different types of filter is in some ways the idea. The combination of a canister filter with an undergravel filter (a "reverse-flow undergravel filter") for example is one of the best ways to maintain Rift Valley cichlids.
Cheers, Neale
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