Hi and welcome to the forum
I would go for a freshwater tank to start with. The fish are less fussy about food and you can have live plants in the aquarium that the fish can graze on. You can have a few types of marine algae in a saltwater tank too but I would start with freshwater.
If you want colourful blue and red fish that live in a habitat that looks a bit like the ocean, then check out African Rift Lake cichlids from Lake Tanganyika and Lake Malawi. The Aulonocara species of cichlid can be quite stunning to look at and you can have rocks and plants in the tank.
Fish can survive a week without food especially if they are fed well in between. So heading off for a week shouldn't be a big issue but start with fresh water, it's easier.
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Generally the bigger the tank, the most stable the water conditions, and this would be safer if something goes wrong while you're away. An aquarium that is 4 foot long would be a good starting point. It is big enough to turn into a marine tank later on, and big enough to give you a variety of freshwater fishes if you like. And if something goes bad while you're elsewhere, the water quality shouldn't deteriorate too quickly.
If you want a show piece, get a tank, cabinet and hood, or just a tank on a metal or wood stand. Set it up and add a heap of live aquatic plants. Let them grow for a few weeks then add a small group of fish. Let them live in the tank for a month before adding more fish. Build the number sup slowly over time so the filter can develop enough beneficial bacteria to help keep the water clean.
You can also do a fishless cycle where you add ammonia and wait a month or so, then add fish. However, you need to add ammonia on certain days and if you are in the air, that won't always happen and the cycling process could stall or take a really long time. So doing a fish in cycle with lots of plants would be my choice.
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What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Depending on what the GH of your water is, will determine what fish you should keep.
Angelfish, most tetras, barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
Having said this, you can modify water by adding mineral salts to make it harder or using distilled, reverse osmosis or rain water to make it softer.