Fallow period

reefer10 · 2026-08-19T15:25:53-0400

How long do you leave a tank fallow for if you do not know the possible disease or cause of fish loss?

vetteguy53081 · 2026-08-19T15:29:33-0400

reefer10 said:
How long do you leave a tank fallow for if you do not know the possible disease or cause of fish loss?

Recommended: 45-60 days in which I refer the full 60 days

JumboShrimp · 2026-08-19T15:40:57-0400

I vote "60" days if you really want to sleep well. (Other people can live with shorter times-- raising the tank temp, etc. But I had one tank that had to go fallow 3 times... likely due to 'user error'... mine.)

MnFish1 · 2026-08-19T15:51:45-0400

reefer10 said:
How long do you leave a tank fallow for if you do not know the possible disease or cause of fish loss?

I personally had this happen (though I think it was velvet). I left it 4 months, while I re-thought what I wanted to do with regards to preventing the problem going forward, as well as making sure parameters were in line. I would think the 60-76 day period is what most people would say. After the 'disaster' - I wasn't feeling any rush to restock.

Jay Hemdal · 2026-08-19T16:33:45-0400

reefer10 said:
How long do you leave a tank fallow for if you do not know the possible disease or cause of fish loss?

That's a tough one. The fallow period for Cryptocaryon, marine Ich is very commonly listed as "76 days", but that is actually an erroneous extrapolation from older literature. Certainly, longer works just as well as the minimum time, but 60 days is fine. It can be as short as 45 days at 81 F. or higher temperatures.

The issue comes in here if you don't know the disease. The fallow period for flukes is 30 to 35 days with no fish host, and for velvet, it's reported to be as short as 24 days, but I use the ich time frame for velvet as well. The issue is really with rare oddball diseases - viruses, internal protozoans and non-obligate parasites. Nobody really knows the time frame for those.....

The biggest issue with "fallow periods" is when new fish are added back into the tank that have not been given a comprehensive quarantine - they can bring a new disease into the tank, making the fallow period *seem* to fail....

reefer10 · 2026-08-19T19:31:58-0400

Thanks for the replies everyone!

reefer10 · 2026-08-19T19:49:02-0400

So it goes like this. I got a lawnmower blenny, rarely saw him nipping at hair algae, and could not get it to eat any other food. So I think he starved, had a very pinched stomach after about a month of no eating. No spots or scratching, or even labored breathing. That fish was only in the tank no more than 5 weeks. Within a week my other fish that I had for about 3 years by themselves and only other fish in the tank, stopped eating as well. The smaller fish a couple weeks prior to that looked like it had lymphocistis, didn’t scratch or flash and ate well, also got better a couple weeks before stopping eating. That fish after not eating for about a week and a half, started breathing heavy kinda swimming a little aimlessly, and hiding a bit. The last 2 days definitely labored breathing and just kinda floating around and laying on the bottom the last day. Then a week later the same scenario that happened with the second fish happened with the other fish. This fish did not show signs of lymphoma. I only saw a very small area of whitish spots on the last fish, but did not look like ich or velvet. So I did a dose of Prazipro. The last one still died on the 4th day of Prazipro. So the last 2 fish went from healthy to what I explained in about a week and a half timeframe. So I don’t know if it was internal parasites or not. Will going fallow kill off that stuff as well?

Jay Hemdal · 2026-08-20T08:35:17-0400

reefer10 said:
So it goes like this. I got a lawnmower blenny, rarely saw him nipping at hair algae, and could not get it to eat any other food. So I think he starved, had a very pinched stomach after about a month of no eating. No spots or scratching, or even labored breathing. That fish was only in the tank no more than 5 weeks. Within a week my other fish that I had for about 3 years by themselves and only other fish in the tank, stopped eating as well. The smaller fish a couple weeks prior to that looked like it had lymphocistis, didn’t scratch or flash and ate well, also got better a couple weeks before stopping eating. That fish after not eating for about a week and a half, started breathing heavy kinda swimming a little aimlessly, and hiding a bit. The last 2 days definitely labored breathing and just kinda floating around and laying on the bottom the last day. Then a week later the same scenario that happened with the second fish happened with the other fish. This fish did not show signs of lymphoma. I only saw a very small area of whitish spots on the last fish, but did not look like ich or velvet. So I did a dose of Prazipro. The last one still died on the 4th day of Prazipro. So the last 2 fish went from healthy to what I explained in about a week and a half timeframe. So I don’t know if it was internal parasites or not. Will going fallow kill off that stuff as well?

That's really tough to say - not sure what you had going on there. The blenny seemingly brought something into the tank, but it also had the common symptom of starving to death. Lymphocystis really only shows uop in newly acquired fish.

This sounds like either uncontrolled, long term marine ich, or gill flukes. Leaving the tank fallow for the 60 days would be your best bet.

reefer10 · 2026-08-20T11:41:00-0400

The white spots the fish I had in there the longest every once in a while would get white spots, not really like ich, the spots were bigger. But when I would do about a 30% water change it would normally get better. Especially if so did the water change a couple times in a week with Prazipro. Sometimes just a couple water changes in a week would help. This time though it did not.

Jay Hemdal · 2026-08-20T11:42:46-0400

reefer10 said:
The white spots the fish I had in there the longest every once in a while would get white spots, not really like ich, the spots were bigger. But when I would do about a 30% water change it would normally get better. Especially if so did the water change a couple times in a week with Prazipro. Sometimes just a couple water changes in a week would help. This time though it did not.

What species of fish would get the larger spots? There is an issue known as "mucus plugs" that causes larger white spots on tangs and some other species when they are stressed.

reefer10 · 2026-08-20T11:43:17-0400

And the spots they did her would not be all over the fish. It was just in an area or 2 on the fish. That’s why I thought flukes. One of the times before this last time, the fish would occasionally look like it was coughing kinda. Thought that was a sign of flukes?

reefer10 · 2026-08-20T11:46:16-0400

They were tank raised true percula clowns. When I first got them years ago they had these spots for a month or so. Treated with Prazipro for a full treatment of 3-4 consecutive treatments in less than a month and good sized water changes. Every once in a while they would show these symptoms just not as bad. Water changes normally fixed it.

reefer10 · 2026-08-20T11:59:01-0400

Not sure if this is telltale of any disease specifically, but the last day when the perculas were alive and breathing very labored. The head and especially the gills/throat underneath was very pale in color.

Jay Hemdal · 2026-08-20T13:45:15-0400

reefer10 said:
And the spots they did her would not be all over the fish. It was just in an area or 2 on the fish. That’s why I thought flukes. One of the times before this last time, the fish would occasionally look like it was coughing kinda. Thought that was a sign of flukes?

Coughing can be a sign of gill flukes, but also can be from too many particulates in the water, or even other gill diseases like amoeba. Scratching the gill converts no surfaces AND coughing is more certainly gill flukes.