Time to spend more money!
Sufficent water movement is indispensible! Sessile invertebrates need this to grow normally, obtain food and to rid themselves of their waste products. A constant one-sided flow is not healthy. Fishes and other invertebrates also react positively to water movement. Of course a seahorse or a jellyfish aquarium has to be designed around different parameters, but that is well out of the scope of these texts. The latest pumps allow varied strength and directions, are not too costly and relatively simple to set up. There are also 'waveboxes' which are designed to simulate natural sea movement.
Filtration is used to process waste products and in some cases to remove them as much as possible before or after they break down. Normal freshwater filtration relies on oxidising the nitrogen- containing substances via nitrite to nitrate. This remains in the water and will be partially removed during regular water changes. Freshwater fish are, with certain exceptions, although sensitive to nitrite, relatively nitrate- tolerant. Plants may also uptake some waste products. So in general an internal filter or external canister works well enough. Unfortunately for us that doesn't work so well in marine systems
LEDs There has been a complete change in aquarium lighting over the last few years. As in previous lighting changes, this was instigated by hobbyists trying out new developments, with lighting units not really designed for aquarium use. The first LED lights were adapted, made water- tight and tested. What failed was lighting with the correct colour temperature and spectrum range. The first commercial LED lighting was not initially trusted by the larger, rather conservative aquarium community, but slowly with specialised LED units fitted in appropriate housings they have taken over the market. A huge advantage also being the much reduced electricity bills in relation to the previously ubiquitous HQI lighting.
HQI- Was the brightest and the best for a reef. The small tubes had to be changed annually at high cost and a normal system might have 2 x 250W on 8-10 hours daily. That's then 5 KWh daily, excluding loses in the control electronics, and the heat produced was excessive, so one but not the only reason for open systems.
Along with blue actinic fluorescent tube(s) on 12-14 hours. So another 1 KWh!
Fluorescent tubes are still available as relacements, but can also be replaced with more efficient, longer-lasting LED equivalents which fit in the old tube sockets.
It was common to use ion exchange resins in the earlier days, these are, however, more use in softening freshwater, replacing mainly Calcium by Sodium.
Today, it is more common to use Reverse Osmosis. These units have come down in price and are more useful in that they remove most dissolved substances, whatever they are. Domestic units run off the pressure of the water supply, which pushes the small water molecules through a fine filter membrane. All metals and organic molecules are significantly larger, can't get through the membrane, and are rinsed away by the water which flows across the outer surface of the membrane. So the unit has one inlet connected to the water supply, and two outlets, one with the RO Water, and one with the water containing the separated substances. Should you not wish to waste this water, it may be used for toilet flushing and other household purposes.