DIY Mason Jar Aquarium

There is a particular kind of restlessness that arrives in early summer, when the light is good and the sea is a long drive away. The cure we keep coming back to is small, cheap and sits on a windowsill: a jar with water in it.

A mason jar aquarium is not a fish tank. That distinction matters and we will come back to it. What it is, is a tiny closed-ish water garden — gravel, water, a plant or two, maybe a shrimp if you commit properly to the maintenance. It takes about twenty minutes to assemble and roughly five minutes a fortnight to keep alive.

What you need

• A wide-mouth preserving jar, 1 to 2 litres. Wide mouth matters — you will need to get your hand inside.
• Aquarium gravel or coarse washed sand, about 3 cm deep.
• Dechlorinated water. Tap water left standing 24 hours, or treated with a few drops of aquarium dechlorinator.
• One low-light aquatic plant. Java fern, anubias or a bunch of hornwort are all close to unkillable.
• Optional: a small piece of driftwood or a clean pebble to anchor the plant.

Total cost, if you already own the jar, is around the price of a takeaway coffee and a sandwich.

Building it

Wash everything, but not with soap. Rinse the jar with hot water only. Soap residue is the single most common reason these fail in the first week. Rinse the gravel in a sieve until the water running off is clear — this takes longer than you expect, usually three or four rinses.

Lay the base. Tip the gravel in and level it with the back of a spoon. Slope it slightly higher at the back if the jar will sit against a wall; it reads as depth from the front.

Anchor the plant before you fill. Java fern and anubias should never be buried — their rhizome, the thick horizontal stem, rots if you cover it. Wedge it between pebbles or tie it to a piece of driftwood with cotton thread. Hornwort just floats, which is fine.

Fill slowly. Rest a spoon or a folded paper towel on the gravel and pour onto that. Pouring straight in kicks up a cloud that takes two days to settle.

Leave it alone for a week. The water may go slightly cloudy on day two or three as bacteria establish. This is normal and it clears on its own. Resist the urge to change the water, which resets the process.

About the fish question

Every version of this project that has ever been posted online attracts the same comment, and the comment is right: do not put a betta, a goldfish or any other fish in a jar. A 1-litre unfiltered, unheated container cannot hold stable water chemistry or temperature, and a fish in one is a fish that is slowly being poisoned by its own waste.

If you want something alive and moving, a single cherry shrimp or a couple of ramshorn snails is genuinely fine in a 2-litre jar with plants — they produce a fraction of the waste and tolerate the temperature swing of a windowsill. Even then, plan on a 30% water change every two weeks with dechlorinated water at room temperature.

Our own preference after years of these: plants only, and let the jar be a green thing rather than a pet.

Keeping it alive

• Light: bright, indirect. Full afternoon sun turns a jar into an algae farm and cooks anything in it.
• Top-ups: water evaporates. Add dechlorinated water back to the original line every week or so.
• Algae: if the glass greens up, you have too much light. Move it a metre back from the window before you start scrubbing.
• Trimming: hornwort in particular will grow into a hedge. Cut it back and start a second jar with the offcuts.

Why it stuck around

This project has been on Homemade Ocean in one form or another since 2013, and it keeps earning its place because of what it teaches rather than what it makes. It is a five-pound lesson in patience: you assemble it, and then the only correct action for seven days is to do nothing. Almost every larger project on this site rewards the same instinct.

If there is no ocean in sight, a jar on the sill is a reasonable start.