I've asked for advice on here a few times and gotten good answers, especially from okiemavis, thank you. So I thought I'd try to contribute something. Here is a cheap, easy DIY idea for a tank with smaller fishes. I have been doing this for a couple years now. This is probably best done outside.
I have a 55 gallon with several species of small schooling fish only in it. Corys, small tetras and rasboras and some red shrimp.
2 trash cans
fine mesh net
small bucket for water, I use a 1 gallon ice-cream bucket
a few cocoanuts
fill the cans with water and add dechlorinator.
drain and split the cocoanuts in half or just break them apart
chip the 'meat' out of the cocoanut and throw it in one of the cans it should mostly sink. A wood chisel works well, but I'm sure a screwdriver or knife would work too. Leave the lid off.
throw the meatless hulls into the other can and put the lid on it
after a few weeks both cans will have started to undergo some changes. In the one with the lid on it, the water should have turned dark brown, the more cocoanuts you used, the darker. This water is highly acidic, and adding a small amount to a tank will lower the KH and Ph. Because a little goes a long way toward lowering them, you shouldnt have to use enough to discolor the water. Just add it bit by bit and keep testing so that you dont overdo it. If you accidently overdo it, you can add baking soda to bring it back up.
In the one with the cocanut meat, there should be a thin film over the top of the water, and as long as it isnt winter, it will be teeming with life. Siphon some water out of the fish tank into the small bucket and then swish the fine meshed net around in the trash can water and put all the little bugs in the small bucket and dump that water into the tank. A regular green or red net lets a lot of the good stuff get through, I cant remember the brand of the net I use, but its blue and I remember it specifically said 'fine mesh' on the package. I mostly get 5 main types of bugs in mine. Lots of mosquito larvae, some little tiny pill bug things that swim, some little things that look like mini-centipedes who cant swim, but crawl around underwater on the sides, some small clear/white worms that are maybe an inch or two long and very thin that swim around like a tiny snake about the size of a hair that swims I guess, and some white things that wriggle slowly and dont seem to move around with any purpose, they just wriggle. They are a little smaller than a full grown mosquito larva and slowly but constantly wriggle. Anyway, all of the bugs are small enough that even little tetras like neons and rummynose can easily gobble up, and they live underwater so you dont have to worry about them dying too soon. All the little fish in that tank go absolutely berserk for the live food. They like it way better than frozen food like daphnia and cyclops and of course way better than flakes or micro pellets. Once the food is all eaten they will troll the top and bottom of the tank for at least another hour scouring for any little bugs they might have missed. The mosquito larvae dive when they get scared and may get in the substrate, but dont worry, they breathe air through a sort of snorkel they have and will eventually have to make a break for the surface, if the corys dont root them out.
I know they warn against this type of thing because of diseases and whatnot, but I have never noticed any adverse effects from doing this. In fact, it seems to stimulate all of them to breed and they have much healthier appetites even for flake and frozen foods when they get an occaisional live meal, their appetites and breeding activity decline during the winter when they dont get their bugs. They seem to breed fairly often in the acidic water with live food. I have no interest in raising them so I just let them eat the eggs. That isnt to say that your results wont be different, so do this at your own risk is all I will say. When doing this, there is usually a small amount of crums or junk from the water that gets netted as well as the bugs, not a lot though. The fish dont seem to like it, but the shrimp seem to think it's caviar.
Anyhow, this is a super cheap, easy, and maintenence free way to get the acidic water that so many fish like, and get a live meal for the smaller fish. The 'bug can' has a slight odor, but nothing unbearable and I cant smell it unless i lean right over the water. I usually 'harvest' my crop of bugs once or twice a week. You dont want to wait too long or the larvae will turn into mosquitos, but if you do it too often you will get all the eggs out and only get little larvaes.