First of all, I'd like to welcome you to TFF
Secondly, take a look at this...
http/www.fishforum...esource-center/
It's our very own beginners start-up guide. The sooner you understand the nitrogen cycle to a certain degree, the easier you'll find fish keeping; you will know what to do in certain situations. Ideally, you want the biggest tank you can afford/have room for. The larger body of water you have, the easier and more stable the water conditions will be. Most freshwater fish available are relatively cheap, it's just the equipment that gets expensive. Try Ebay for your tank but get everything else relatively brand new in condition purely to ensure it's long life and so you have a warranty on the most important bits like the filter and heater.
Is the budget of £100 for a tank minus the equipment or a tank
including the equipment?
Equipment:
Tank
Substrate (do you want real plants? Their benefits are unrivaled in the fresh water aquatics world in my opinion but requires some more research and a little extra money for basic fertilizers)
Decorations (natural or fluorescent styles are very common, which do you prefer?)
Heater (on TFF we tend to go by 1 watt per litre of water - you don't even need a heater if you don't want tropical fish)
Filter (I would personally recommend not even opting for an internal and spend the extra £25 if available on an external canister style filter - see Eheim, Fluval etc, it'll save you a lot of time on maintenance)
Where feeding is concerned, I feed all my fish on a tri-weekly basis (3 or 4 times a week, once every other day or there abouts) and regardless of the large 40% weekly water changes I do, all my utility bills are pretty much unchanged. The filter, heater and powerhead (a pump used to increase water flow) use up a minimal amount of electricity; they are on 24/7. I spend maybe £10 a month on fish food and £20 on top of that every other month for fertilizers for the plants. Healthy fish take care of themselves once fed and water conditions are maintained. Fish also live a lot longer than most people anticipate. Take the average goldfish - if it was properly looked after in a good size aquarium, it'd live as long as 10 years maybe!
Once we know what kind of equipment you're getting and what dimensions your aquarium has, we can suggest what fish you can facilitate. Tall tanks are generally not a great option to begin with as problems with oxygen in the water can arise but longer, more cuboid style tanks are considered a good starting point.
The most important thing to remember is: fish will die but let it be a learning curve, there is a fair amount of info to understand but once that bit's over and done with, it's plain sailing - don't let anybody put you off!