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Many systems can benefit from a deep cleaning once a year or twice a year. In order to get in there and get all the muck out, some owners and maintenance companies will remove the fish and put them in a temporary holding tank. Before you toss the fish back into the clean system, keep the following two points in mind:
1. pH Crash
When you have to catch up lots of fish and put them in a small tub, they will get stressed out. Stress causes additional respiration, causing a respiratory acidosis in the holding tub. If there is enough buffers to go around, a measurement of kH, your pH should not change much. If fish are held in their tub for awhile, the water pH can drop significantly.
Now your main pond has been cleaned and is all ready for the fish.
Check your pH in your main pond and holding tub to make sure they match. They should be at least 0.2. If you toss your fish in holding from 6.0 to 8.0, you will kill them. This is the #1 cause of death after deep cleanings. This is also the reason we don’t recommend 100% water changes for ANY fish system.
If your pH does not match, gradually add new water to the holding tank. Be sure to add your conditioner! Once the pH’s match, you can move your fish, provided point #2 is also in place.
2. Temperature Shock
Some fish, like koi and goldfish, can tolerate a wide range of temperatures. What they can’t tolerate is a sudden change in temperatures. Moving your fish from a 75F holding tub to a 60F pond can be exceedingly stressful. Although it is unlikely to kill instantly, your fish will be weakened and susceptible to secondary infections.
Like your pH, make sure your temperatures match within 4 degrees before moving your fish back to the pond. Again, using gradual water changes in the holding tub will help correct any discrepancies.
If you are entrusting your deep clean to a maintenance company, they should be able to explain the above two concerns to you. If not, hire someone else.