Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Most nitrifying bacteria live on surfaces, not free in the water, so 100% water change means almost no bacteria are lost.
Drying out the bacteria is bad, though, so if the tank was drained, then sat a few days, that could make the cycle restart.
The lower mineral level may not be quite so good for the bacteria, but as long as it is not pure RO water they should do OK.
Remember that when you are doing the fishless cycle the goal is to grow the bacteria as fast as possible. You want to give the bacteria optimum conditions. After the colony is well established you can change the water to suit the fish, shrimp and plants. The bacteria colony may slow down, but that is OK. You are no longer in the 'Grow it as fast as you can' phase, and the plants will take over some of the water treatment duties.
If the cycle seems stuck, here is what is often going on.
The ammonia to nitrite bacteria are fast growing. They will be able to remove quite a bit of ammonia (3-5 ppm) within a week to two weeks of starting a fishless cycle.
The nitrite to nitrate bacteria (Nitrospira) are slower growing, and more particular about the conditions.
So the first group of bacteria are busily changing ammonia to nitrite, but the nitrite bacteria are just sitting there, doing almost nothing.
These bacteria do not grow well when the nitrite or ammonia are too high. Max is 5 ppm. So, if the nitrite is rising too high, do a water change and add only enough ammonia to test 3 ppm.
Other optimum conditions:
High GH and KH. These bacteria need some minerals. In fact, I would add some plant nutrients, too. KH2PO4, trace minerals.
High pH. For growing them as fast as possible, a pH in the upper 7s is good. If the pH drops to 7.0 they will still do OK, but under 6.5 they really seem to struggle.
High oxygen. Good water movement.
Moderately warm. Upper 70s F would be good. Higher is OK if you can keep the oxygen level up. These bacteria will even grow at room temperature, and in an outdoor pond, but the cooler it is the slower they grow.
Repeat: Set the conditions for fastest growth during the fishless cycle. You can alter the conditions after the colony has grown.
A fishless cycle with plants can be different. The plants are part of your bio-filter, so the nitrogens (all 3) may not test the same as in a non-planted fishless cycle.
Go by the test results:
When the tank, the system as a whole, can remove 3 ppm ammonia within 24 hours, and the NO2 is zero at 24 hours, then the tank is cycled. It does not matter what the NO3 is. It does not matter if the plants are doing most of the work. If the plants and bacteria can remove the ammonia, and not show nitrite, then that part of the system is safe for fish.