If you wish to keep your bait alive for long periods then an aquarium is a good way of doing it. It is also a good way to keep microspecies (e.g Blennies, gobies) or immature fish for a while. Children can also use this set up to keep creatures they find in rockpools before you pinch them for bait. I have tried to simplify things as much as possible since most textbooks over complicate things. This is not a complete how to guide but covers the main points, a general fishkeeping book is worth buying to help with the water chemistry. But choose carefully as many again overcomplicate things, you do not require a doctrate in marine biology to keep fish, it is well within the reach of most children never mind adult anglers.
Buy a tank. The bigger the better. I recommend no less than 120cm in length. Anything smaller and it will be difficult to maintain water quality. This also gives enough room for the more terrirtorial inhabitants. I have lost expensive fish to bullying so I speak from experience.
Clean the tank. A quick rinse if it is brand new a complete disinfection if second hand. People often sell their tanks when they have lots of fatalies. (These are often due to various diseases many of which are impossible to cure and you need to start again. This can be expensive never mind time consuming so bye bye tank). There are many ways of disinfecting a tank. Add 50 ml of 30% hydrogen peroxide to 100 litres of water and leave in tank for three days, WITH LIGHTS ON. After this time most of the hydrogen peroxide will have decomposed to water. Rinse. Alternatively use Potassium permanganate. Fill the tank and add enough to get a strong violet colour, leave for three days and rinse clear. Or try spraying the tank with 70% Isopropanol. Cheap skates can wash the glass with a slurry of salt and water and rinse after a wek. Alternatively visit the pet shop and buy something off the shelf.
Lay a undergravel filter on the bottom of the tank. Fix it to the bottom using a silicon glue recommended for fishtanks. Lie a sheet of fine gauze nylon net on top and attach either one or two uplift tubes (depending on A or B set up).
Fill with 3 inches of washed coral sand. Do not use sand from the beach. The bacteria which run the tank prefer coral sand.
With set up A an external power filter is used to drive the water through the filterbed (the sand). This can be done in any direction but a reverse flow is recomended because the water is filtered before it goes through the gravel helping to keep the sand clean. It also helps keep small food particles in suspension which pleases the filterfeeders such as mussels. The use of an external filter also allows you to add a UV steriliser at a later date.
With set up B a powerhead is used to suck the water down through the sand and up the uplift tube. It is best to fix one at each end of the tank to get a better flow rate and guard against one failing in the middle of the night. The sand can get compacted using this method so rake over occassionaly to maintain a high flow rate. At the end of the day which method you use is a personal one. Just remember to keep the flow rate high- about 50 litres per square metre per minute (pumps and filters always state flow rate on box, usually with a guide to tank size)
Now for the fun bit.
Apart from making regular water changes to keep nitrate levels low and feeding there is little else to do but enjoy. Workout yourself the amount of water to change and how often according to need, using nitrate levels as a guide (hint. fish are more tolerant than invertebrates). Frozen food for the fish,crabs,shrimps etc and microscopic food for filter feeders can be bought at most decent petshops. A Knowledge of the animals you keep is useful and essential if you decide to keep tropical marines as they can be very expensive to buy.