Hi David,
What's the throughput of that filter? I'm not familiar with that model. A turnover of 4 times per hour is often recommended as the baseline for non-marine aquaria. I'd also downrate the quoted turnover of your filter by a factor of anything up to 50%, to allow for clogged, densely packed media. So a filter quoted at 1000 litres per hour is, in my opinion, realistically one with a turnover of around 500 litres per hour.
Some fish simply like to swim into currents. My halfbeaks do that, and so did Colombian sharks when I kept them. My South American puffer definitely goes through phases where he swims into the stream. I'd actually consider adding aeration devices (like a large airstone) before adding another filter. It will help aerate the water, which puffers appreciate, and since you can position the airstone at the bottom of the tank, you can generate some movement at the bottom of the tank.
I don't actually like airstones because they look unrealistic, but they can be used intelligently. I've seen some outstanding examples of this, such as placing the airstone inside a ceramic amphora or pot, so you don't see the airstone, just the bubbles. A strong pump looks better than a piddling little thing, and wooden airstones are better than ceramic ones. A fine mist of bubbles looks nice than a stream of discrete, large bubbles.
Cheers,
Neale
