what do you want out of your aquarium?
where will you get everything?
what do you already have?
do you have a specific fish, look, biotope, landscape, etc. in mind?
Well I hate to admit this but I spent way more than $300 to set up my "free" 20 gallon...well the tank itself was free and I got everything else brand new. I still have not stopped spending $$$ :nilly:
Sorry for not being specific. The 300g would be a central/south american cichlid tank with driftwood and a few weighted plastic plants, the 180 would be for a dovii, and the 125 would be a general community aquarium. Also, this is just a hypothetical question, I am not planning to get these aquariums ANY time soon.
It really depends on you. If you're lucky or how resourceful you are. A lot of things can be done diy to save some money.
When I got my first 50 gallon breeder for $25 I put ads in craigslist looking for people with unwanted fish . I got 10 free malawi mbuna cichlids out of the deal, and I was able to return to get 10 more for my 55 long. So yea, just do a lot of research.
I spent $45 on my hob filter for the 50, and $10 for a submersible pump from harborfreight. $10 for pool filter sand. I had an air pump. You also have to take into account medications, water conditioners, food.