Buy your aquarium used i have never bought a new tank and have never spent more than 300 on any of my tanks (and i have alot -_-) plus usually when you buy a used tank it will come with everything you need even fish sometimes
To save money on lighting on one of my tanks that came with no light i bought a 48" shop light it cost me $17.00 which i fitted with aquarium bulbs i haven't had any problems yet. You just gotta make sure no water can hit it. My lid was made from food so i was able to make a cut out and put some plexy glass down and set my light on that.
There are always ways to save money on decorations. There are quite a few different things you can do i love some of the custom Styrofoam backgrounds people make. I make my own plants i buy the fake plants at the dollar store pull any wire out take the bottom of a 2L pop bottle put a hole in the centre put the ends of the plants through it and use a glue gun to keep it all in place for smaller plants i use the bottom of a water bottle.
I find rocks out side which i wash and place in my tank. I buy figurines and ceramic pots at the dollar store and put them in my tank too.
For my hob
filters to save money i buy dehumidifier filter padding at the hardware store and cut it to fit my filters.
Some of my tanks i don't buy aquarium bulbs for if you can find any " white light " bulbs that fit your fixture they are fine to use.