You should be able to manage without a filter. get some oxygenating plants though and change the water loads. also don't go mad on how many guppy's you get. remember the little buggers breed like rabbits on viagra.
have you checked out pets at home? i wouldn't recommend buying the fish there judging by the ones i've seen but their filters are quite cheap. i bought the biggest internal fluval there for about £25. if i remember rightly the fluval 1 was only about a tenner. that might be wrong though
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even for a small tank i would be inclined to get a filter - it just gives you a wider margin for error with stocking/feeding/water changes/ oxygenation etc etc. however with a bit of work you should manage without one.
regards
have you checked out pets at home? i wouldn't recommend buying the fish there judging by the ones i've seen but their filters are quite cheap. i bought the biggest internal fluval there for about £25. if i remember rightly the fluval 1 was only about a tenner. that might be wrong though

even for a small tank i would be inclined to get a filter - it just gives you a wider margin for error with stocking/feeding/water changes/ oxygenation etc etc. however with a bit of work you should manage without one.
regards