To loosen and improve aeration and drainage of commercial potting mix or seed-starting mix, combine it with screened pine bark mulch. Bark is cheaper and coarser than Perlite or chicken grit. It retains and wicks water better than crushed rock.
You can use any ratio depending on how fast you want it to drain and how much air you want for your roots. Maybe just add 20% - 30% to your current potting mix to reduce its cost and make it go further. Maybe add just a little commercial mix to a lot of screened bark mulch, and fill lots of 5 gallon buckets economically.
Grit and crushed rock to improve drainage should be around 1/8 inch, say 2-3 mm or even 3.5 mm. But even sharp crushed rock grains are only slightly irregular - like marbles. Pencil shaped grains or tongue - depressor - shaped grains would open up the soil-less mix better.
Pine bark shreds should probably be 1-3 mm in their thinnest dimension, but their length can be anything that fits in your pot or cell - 1/4" to 1/2" or even longer, if you can get them. Long shreds really help keep air channels open! Bark shreds plus some grit or coarse Perlite make well-aerated, long-lasting potting mixes, as long as you don't choke them by adding more than 10-15% fine stuff.
Discard all bark chunks too big to pass through ½" hardware cloth - those are much too big for starting seeds or small pots. Instead use the chunks outdoors as top dressing (mulch). Or chop them up and re-screen them!
Probably discard most bark chunks too big to pass through 1/4" hardware cloth, or only use them in large containers like 5 gallon buckets that need a very open mix. Those are too big for starting seeds or small pots.
Discard some of the bark fines that pass through through ¼" hardware cloth, if you start with dusty mulch and only have 1/4" screen. You do want some bark grit around 1/10th inch and a little down to 1/16th inch, but not a lot. Your water retention will come from the fine stuff, and even dusty powder, but that fine stuff also decreases aeration.
If you have 1/8" mesh hardware cloth, you want anything that passes through 1/4" mesh, but is held back by 1/8" mesh. You may also want some of the stuff that passes through 1/8" mesh, for more water retention and less frequent watering. Not the bark-dust. You want the bark-grit, not very much smaller than BBs.
It also matters how big your pots are. You can use bigger bark nuggets in a 5 gallon bucket or 1/2 gallon pot than you can in an 8 ounce Dixie cup or 72-cell seedling tray.
(If you are starting very fine seeds like petunia or lobelia, that need light to germinate, this very fine "bark powder" could be used as the top layer of seed-starting pots or trays. It's like peat, just not as prone to hold excess water and doesn't break down as fast. And it re-hydrates easier if it gets totally dry. Or vermiculite works great as a thin top layer to support tiny seeds.)
To reduce the amount of bark fines ("powder") that you have to screen out, start with medium mulch, not fine mulch . "Fine Bark Nuggets" are a better starting point than "Medium Bark Mulch" because nuggets tend to be cleaner and less dusty than mulch.
If you use more than 10-15% small bark fines, sand or peat in your mix, they would sift into and close up any air channels or pockets you created, defeating the goal of fast-draining, well-aerated potting mix or seed-starting mix.
Don't buy mulch that was stored wet in a plastic bag if it smells bad - anaerobic fermentation products are acidic and bad for root hairs. If you must use "smelly bark", flush it with water and let it air out well first. Or, better, compost that smelly mulch or just use it AS mulch or a soil amendment in a raised bed outdoors.
Pine bark is best. Fir or hemlock bark is good, too. Lignin keeps them rigid, which promotes air channels in the root zone. Suberin, a lipid in conifer bark, helps them resist decomposition for several years.
It's worth your while to pay $4.25 for 2 cubic feet of clean,. dry bark nuggets at Lowes. That's only 28 cents per gallon.
Maybe even "splurge" $7-8 on a 2 cubic foot bag of GOOD, dry bark mulch from a good nursery, if you find a brand that has longish chips and shreds but no powder. 53 cents per gallon is cheaper than most mixes.
The $3.50 bags of "mulch" at my local Home Depot have usually been stored wet, and have too much dust, dirt and wood. I call those "logyard trash".
If you do want a fine seedling mix that holds a lot of water, with only so-so aeration, mix 4 parts of clean pine bark shreds (no powder) with one part of fine, powdery commercial peat-based mix. Or better-quality seeding mix that has airy structure.
The 20% "fines" will provide a lot of wicking and water-retention, but hopefully not choke off ALL the air pockets. Better to start with a better, coarse-fiber commercial mix, and still add lots and lots of bark shreds or nuggets.
Not many people use bark for seed starting. Its usual use is potting mix and large nursery pots. But it certainly has improved my success at starting seeds indoors.
It's great for top-watering, and it's a life-saver for people who are always tempted to over-water. I haven't seen any damping off at all since I started using it, and seedling roots penetrate all the way down to the bottom of shallow cells or pots.