A jar in trouble rarely has one problem. It has a chain: too much food leads to high nutrients, which leads to algae, which shades the plants, which lowers oxygen, which stresses the shrimp. If you treat the last symptom first, you usually make the chain worse.
The skill is to find the earliest link and change one thing.
Step 1: Stop adding things
Before you test, clean, or dose anything, stop feeding. Do not add animals. Do not add chemicals. Do not change all the water. A jar in trouble is a system out of balance, and every new input is another variable.
Step 2: Look at the four basics
You do not need a laboratory. Four observations cover most problems:
Oxygen. Are the animals at the surface? Are they breathing quickly? Is there a film on the water? Low oxygen is the most urgent problem because it kills fastest.
Light. How many hours? Is the jar getting direct sun? Has the season changed? Too much light is the most common cause of algae.
Waste. Is there uneaten food on the bottom? Dead leaves? A dead snail? Detritus is food for algae and a source of ammonia.
Plants. Are they growing, or melting? Are they pale, leggy, or covered in algae? Plant mass is the engine of the jar; if it is failing, everything else follows.
If you have test kits, measure ammonia, nitrite, and nitrate. In a small jar, ammonia is the number that matters most after a death or overfeeding.
Step 3: Fix in this order
1. Remove the source
Remove uneaten food, dead plant matter, and dead animals. This is always the first action. A dead snail in a 2-litre jar can produce enough ammonia to kill shrimp within a day.
2. Restore oxygen
If the animals are at the surface, do a 30% water change with dechlorinated water at room temperature. Increase surface movement: pour the new water from a height, or add an air stone on low for a few hours. Reduce feeding. Do not add more animals.
3. Reduce light
If algae is the main problem, cut the light to six hours a day for two weeks. If the jar gets direct sun, move it. Do not clean the glass and increase the light at the same time; that removes the algae that is currently using the nutrients and gives the next bloom more light.
4. Change some water
A 20–30% water change removes nutrients and dilutes ammonia. It does not fix the cause, but it buys time. Use dechlorinated water at the same temperature. Do not change more than half at once; a large change can shock the animals and reset the biology.
5. Add plant mass
If the jar has room, add a fast-growing plant such as hornwort or water sprite. More plant mass means more oxygen and more nutrient uptake. Do not add plants that need more light than the jar has.
6. Wait
After one round of fixes, wait a week. A jar needs time to respond. Changing something every day is its own kind of damage.
What not to do
- Do not change all the water. It removes the bacteria and biofilm the jar depends on.
- Do not scrub everything. Cleaning the glass, the substrate, and the plants in one session removes the food the shrimp need and can trigger a new bloom.
- Do not use algaecides. They are hard to dose in small volumes and toxic to invertebrates.
- Do not add a fish to eat the algae. Fish add waste and make the problem worse.
- Do not add “good bacteria” bottles to a jar. The jar’s surfaces are already colonised; a bottle does not fix an oxygen or nutrient problem.
When to start over
A jar is worth restarting if:
- The substrate has gone anaerobic and smells of sulfur.
- A disease or parasite has spread through the animals.
- The jar has been unbalanced for months and every fix has failed.
- You want a different shape or stocking plan.
Before restarting, save what you can: healthy plants, a few shrimp, and any hardscape. Rinse them in dechlorinated water and move them to a clean container. Then rebuild slowly, with less stock and more plants.
The post-mortem
After a crash, write down what happened:
- What did you add, and when?
- How much light, and for how long?
- When did the water change?
- What was the first visible sign?
The pattern is usually obvious in hindsight: a new animal, a bigger feed, a sunny week, a forgotten light. The next jar is easier because you know which variable you are likely to push too far.
The Pondlet simulation is a low-stakes way to see the chain: overfeed, watch nutrients climb, watch algae follow, then change water and light and watch it recover. The numbers are simplified, but the order is real.