Walstad Shrimp Tank

A 6.5 gal will provide plenty of food for shrimp if you wait a while for algae to grow in and keep it on heavy light to help speed that up. Sunlight is great for self-contained tanks. The more surfaces in the tank the more can grow, though the effect isn’t that extreme. Incidentally with a bottle like that, don’t fill it up all the way to the neck, leave as much surface area on the water as you can still available for use by plants and for gas exchange.

If you want snails in there maybe consider how much competition with the shrimp they’ll be. If you put a species in that breeds easily don’t be surprised if they beat out the shrimp long term. I wouldn’t recommend fish because frankly, how the hell are you going to get the fish in or out of there without pouring the entire thing out? It’s possible certainly, but maybe something you could access with a net would be better for a fish unless you’re happy with it spending the entire rest of it’s life in there and with much more limited ways to intervene if it gets sick. You’ll also probably need a syringe if you plan to do water tests in that jar. I’m not saying “Don’t do this” if it’s what you really want, but you should probably have a plan for this.

Hornwort doesn’t compete with duckweed significantly. Hornwort grows to the surface but breaches it poorly, and generally the duckweed will just grow above it like that. Also hornwort is crazy easy to propagate manually, just cut off literally any piece of it with a couple of leaf rings – It also spreads laterally through branches constantly, and it will often start these branches so low down they’re practically runners like in vals. Also to be frank, duckweed tends to outcompete practically everything else anyway… they don’t call it aquarium herpes for nothing.

You’re overthinking the substrate but mixing crushed shells in it is a pretty good idea, as calcite will generally maintain your water hardness reliably at at least 7.4-7.6. Limestone, coral and aragonite sand will do the same thing. Carpets won’t bother anything you’ll put in there and are good use of the space for your purpose of trying to reduce any maintainence on this tank. If you wanted to provide calcium for the shrimp remember they don’t really need all that much and their moults recycle what’s there, and you could just put a piece of a calcium carbonate indigestion pill in there for that anyway, you don’t need to rely on the substrate.

The most difficult part of this for you in the long term might be doing trims. To ensure a walstad tank continues to continually remove nitrates and ammonia from the water, you have to trim the plants enough to ensure perpetual growth. Your aquascaping tools should help, but expect it to be a bit difficult – Of course you could always use a turkey baster to hoover up duckweed.