I've used a pair of Emperor 400 filters on a 75 gallon tank continuously for the past five years or so. So far, neither has needed attention other than filter changes and recently, new impellers. After five years, the constantly spinning impellers had worn their shafts and bushings to the point where they made a rattley noise, although they still moved water effectively. Replacing the impellers was an easy repair, and restored the unit to almost total silence. The filter may actually now be quieter than when it was new, as there appeared to be a slight design change in the new impellers when compared to the old ones. The loudest sound is that of water returning into the tank, which varies from almost inaudible to a gentle splashing sound, depending on whether the water level is above the level of the return chutes or below them. The tank is located in the living room where I watch television, and I'm never distracted by the sounds of the filters.
For almost anyone, a single Emperor would provide adequate filtration, however at the time I purchased them, I had seven huge goldfish in the tank, who produced vast quantities of waste matter. Eventually the goldfish grew too large for the tank and were relocated to a pond in the backyard, and the tank was restocked with a variety of colorful tropical fish.
The big selling feature for the Emperor 400 is the dual biowheels, which provide a highly oxygenated living space for beneficial bacteria to colonize. These bacteria help remove some of the waste products which the fish produce, resulting in healthier fish, and extending the time interval between water changes, and thus reducing maintainence of the aquarium. Other filters have alternative methods of providing biological filtration, but the biowheel is so effective that it can keep nitrates at an undetectable level. The Emperor filters, both the 400 and the 280, have an adjustable spraybar above the biowheels. Water falling onto the wheels from the spraybar helps insure that the biowheels never stop turning, and allows the user to adjust the speed at which they turn. The standard Penguin filters have biowheels, but no spraybars, and so the biowheels only turn by the action of water flowing beneath them, with no adjustment for speed. I'm still using the original biowheels which came with my filters, with no end in sight. I've never even cleaned them or seen any need to do so.
These filters are made of a black, slightly flexible plastic which seems to be less brittle than the transparant material used on some other filters. I used to use the original Marineland brand replacement cartridges, but have since found that some aftermarket filter pads for the Emperor are made with a reuseable frame which snaps together on either side of the filter pad. This allows the user to purchase filter floss in a bulk roll. When the filter pad becomes clogged, I simply cut new floss pads to fit the frame, making the cost of renewing filter pads extremely low.
As others have mentioned, each filter has four slots for filter cartridges. And the Emperor 400 comes with two refillable media containers which can occupy two of those slots, if you choose to use them. They can hold extra carbon, zeolite, or other material. I have mine filled with porous ceramic noodles.
The Emperor 400 has a control for water volume, so you can turn it down when feeding your fish, preventing all the food from being sucked into the filter. After the fish have finished eating, the volume can be set back to "high", however I have left it in the second slowest postion for weeks at a time, and the fish seemed to be content. I have found that if the volume control is not moved for a long time, it tends to accumulate mineral deposits and become stiff. So, I turn it down every day when feeding, then turn it back up a couple of hours later. Also, if the power is interrupted while water volume is turned down, the filter takes longer to prime itself and begin pumping properly. If power fails when volume is turned up, the filter restarts with no problem when power is restored.
The Emperor 400 comes with a cleaning brush for the spray bars and an extension for the water intake to be used if you have a very deep tank. My own tank is fairly deep, and the extension would bring the intake nearer the bottom, however I don't use the extension, and filtration seem fine.
When setting up an aquarium, it should be born in mind that the closer to a natural habitat you can get, the healthier your fish will be and the less maintainence will be required. My own tank is set up as follows:
Two inches of "Eco-Complete planted aquarium substrate" soil purchased in sealed bags at the local store AquariumConnection. The soil is intended for use in aquariums with live plants, which feed on some of the wastes. In the soil, there are approximately 20 Anubis plants scattered around the tank. The fish love to swim between and around the leaves, or hide behind them. From time to time, eggs are laid on the leaves of the plants. The bright green leaves stand out in vivid contrast against the nearly black soil. Two 15 watt fluorescent lights provide illumination to stimulate photsynthesis in the plants.
I strongly recommend that you do NOT use any type of gravel, colored or otherwise, as food falls down into the gravel where fish cannot reach it, and where it rots and fouls the water. If you choose to use gravel, you MUST vacuum the gravel on a weekly basis, and your water still will not be as clean. Gravel equals more work and dirtier water. Sand is better than gravel as food cannot fall down into the sand, but sand still doesn't provide nutrition for live plants. Use Eco-Complete soil or something comparable, both you and your fish will be happier.
I also recommend that you do NOT use any type of undergravel filter, as these will also require frequent vacuuming, and eventual dismantling of the tank to clean beneath the filter plate. With the Emperor, you won't need additional filters. And again, undergravel filters equal more work and lower water quality. I NEVER vacuum the soil, and only perform a 20% water change every couple of weeks. I change filter pads when they become clogged, usually every couple of months.
The fish themselves consist of a number of cory catfish, who snuffle around through the soil and pick up whatever falls from above. There are as well golden tetras, cardinal tetras, redline tetras, silver dollars, danios, flying clouds, and mollies, and one otocinclus catfish. Some of the cory catfish are grandfathers several times over, and have grown to a large size in the five years or so they have lived in this tank.
I feed them once each day, around 6:00 pm. Supper consists of several spirulina wafers, plus a pinch of flake food about the size of a nickle, which I crush between my fingers to produce many very fine particles.
To sum up, the Emperor 400 is an efficient and durable filter which will help provide good water quality and healthy fish.