How long can the fish go without food over a four-day Thanksgiving trip?

You're packing the car for Thanksgiving, and the fish are watching you through the glass the way they do at every feeding. You'll be back in four days. Will they be okay? Yes. Healthy, settled adult fish can go four days without food. In most tanks, the bigger risk is too much food, from a last-minute "feast," a dissolving block or a well-meaning sitter. Below I cover why a short fast is safe, which tanks are the exceptions, and what to do before you leave.

The short answer: four days is fine for healthy adults

A four-day gap won't starve a healthy adult fish that has settled into its tank. Fish are cold-blooded and don't burn energy the way we do. Gregory Lewbart, a professor of aquatic, wildlife and zoologic medicine at NC State's College of Veterinary Medicine, says most fish can survive days or even weeks without food. He said it about power outages, but the biology is the same when you go away on a trip.

Fasting research points the same way. In a controlled study, juvenile leopard mandarin fish went 5, 10 or 14 days without food. None died in any group over the eight-week trial. Once feeding resumed, they caught up to the fish that had been fed all along. The same paper explains one way fish manage this: during a fast they shrink their gut tissue, which lowers how much energy they need. These were farmed juveniles of an Asian food fish, not stream fish in a home tank, so treat the study as supporting evidence. But four days is well short of anything those fish went through.

The extreme case is two goldfish in a Christchurch, New Zealand office tank. They survived 134 days with no food and no power after the 2011 earthquake, probably by grazing algae. Don't use that as a care guide. It does show how much reserve a fish has.

So hunger isn't the real danger over a long weekend. Overfeeding is.

Who should not be left unfed

That reassurance has limits. It covers healthy adults that eat well. Some tanks need a plan:

• Fry and juveniles. Young fish are growing and have small reserves. Don't leave them unfed for four days without a real plan.
• Thin or newly bought fish. A fish that arrived skinny, or hasn't settled in and started eating well, has less to live on.
• Fish recovering from illness or still in quarantine. A fish that's still recovering needs steady care, and a quarantine tank needs watching anyway.
• Heavy grazers in a bare tank. Fish that pick at algae and biofilm all day do better with something to graze. A spotless tank with almost no algae or biofilm gives them nothing while you're gone.
• Tankmates that will eat each other. If a hungry fish would pick off smaller tankmates, four days of hunger can be enough to make it try.

Then there are the residents that aren't fish. In my old River Tank, fish shared the water with little crabs, small turtles and fire-bellied frogs, and chameleons lived in the same setup. These animals have very different needs, and there's no single safe rule for leaving them unfed. Check the needs of your own species before you go. Look up each one separately, and if you're unsure, have someone check on them or feed them.

Skip the feast: do not overfeed before leaving

Most people get this wrong. It feels kind to give the fish a big meal before you go, but it isn't. Fish eat what they want in a few minutes, and the rest sinks into the gravel, the stones and the corners of your pools, where it rots.

A good rule of thumb is to give as much as your fish can eat in 3 to 5 minutes, once a day. On the last day, feed that normal amount and stop there.

The reason is water chemistry. Food has to be eaten and digested quickly, or it pollutes the tank with ammonia. NC State's vets note that uneaten food adds excess nitrogen and degrades water quality. In a tank you check every day, you'd catch a cloudy patch or a smell. Over a four-day trip, nobody's there to catch it. A pile of food that the fish would never have finished can push ammonia up while the fish themselves are fine without it.

One more reason to keep the last meal normal: if the power goes out while you're away, Lewbart advises not feeding fish at all, because digestion uses oxygen. Without power, your pump and filter stop moving water, and oxygen is already short. Fish that aren't full of food handle that better.

Feeders and vacation blocks: do they help over four days?

Pet store shelves fill up with "vacation" products in the fall. Here's what they are.

Vacation blocks (the smaller versions are often sold as "weekend blocks") go into the tank and release food as they dissolve. They're aimed at smaller tropical fish. The catch is the one above: a dissolving block keeps adding to the water whether the fish eat it or not, and it can pollute a tank that's left alone too long.

Tetra's gel-based TetraVacation works differently. The company says it is plaster-free, doesn't dissolve, and releases food only when fish nibble it. Tetra says it feeds tropical fish for up to 14 days without ruining water quality. Those are the manufacturer's claims, so weigh them that way.

Automatic feeders are hoppers that drop a set amount of food on a timer. People use them on vacation, and they can work. They also have known weak spots. Moisture can get into the hopper, which makes the food clump and can jam the mechanism. That's worth thinking about over a tank with a waterfall that keeps the air above it humid. Fish also learn when the feeder goes off, which can cause feeding frenzies and wasted food. If you want one, run it for a week or two while you're home. Watch how much it actually drops and whether it clumps.

My plain advice for four days: for healthy adult fish, feeding nothing is usually safer than a gadget you've never tested. Save feeders and blocks for longer trips, and try them out at home first.

Pre-trip checklist

Here's what to do before you head out on Thanksgiving week:

1. Do a partial water change a few days ahead, not the morning you leave. That gives you time to see that the tank is stable and nothing went wrong afterward, like a heater you forgot to plug back in or a cloudy tank.
2. Check the filter and pump. Make sure the flow is strong and nothing is clogged. In a river tank, the pump moves the water and carries oxygen through your pools, so it matters more than usual.
3. Check the heater. Late November can be cold, and some people turn down the house heat while they're away. Confirm the heater is working and the tank is holding its normal temperature.
4. Check the lid. I learned fast with my River Tank to replace the light kit with a full screen lid, because crabs and some reptiles get out of almost anything. An escaped crab while you're out of town has no one to find it. Close every gap.
5. Put the lights on a timer. That's my own advice rather than a hard rule. Fish keep a normal day and night, and the lights won't stay on for four days straight.
6. Give the sitter a small job. The safest request is a look-over: are the fish moving, is the water clear, is the pump running, is the heater on? If you'd like the fish fed, portion each feeding into separate small containers ahead of time and ask the sitter to use only those. A sitter guessing at "a pinch" is one of the most common ways a tank gets overfed.
7. When you're back, go back to normal. Feed your usual 3-to-5-minute amount, not a make-up meal. Remove anything they don't eat.

Do these, and the fish will be fine while you're at Thanksgiving dinner.

Sources (6)

• NC State College of Veterinary Medicine: Dealing With Aquariums and Ponds During Power Outages
• PMC: Fasting and refeeding study in juvenile leopard mandarin fish
• Gulf News: New Zealand goldfish survive 134 days without food
• Wikipedia: Aquarium fish feeder
• Wikipedia: Aquarium fish feed
• PetSmart: Tetra TetraVacation Slow Release Tropical Fish Feeder

Photo by geraldrose on Pixabay.