First off, I bought a breeding group to begin with. So for me they have just continued. However, I have also spawned various corys, farolowella vitatta, angels,bristlenose and a few others that respond to similar triggers.
The first thing is to understand how they spawn in the wild and where it is they live. They are almost on the equator in a single river. This area has basically two seasons- dry and rainy.
During the dry season river levels hit their lowest levels and their warmest temps and they will also flow at their relatively slowest rate. This means that food should be easier to find and that the water is dirtier- ie tds are higher as is the pH. The onset of the rainy season is heralded by a change in barometric pressure as storms roll in and the rains come.
Rather abruptly river parameters and conditions change. The influx of new cooler water causes current to increase and water levels to rise. The pH drops as do the TDS. And this is what triggers spawning.
TDS and GH are not the same thing, but GH is included in TDS to which other things contribute. But you can not have high GH without also having high tds and lowering gh will also lower cause TDS to drop.
To get zebras, and most other seasonal SA spawners, to get jiggy normally requires that the fish experience some semblance of seasonality. The reason I say "some semblance" is because there are no hard and fast numbers perse, but ranges that will work. And every group of fish in tanks will have its own way and the fishkeeper will have his or her own water params out of the tap.
The hardest part to spawning zebras is getting them to go the first time. Once they have, they will keep going on their own with much less work on the fishkeeper's part. They will normally spawn for a number of months and then go on hiatus for a number of months and then resume. They are seasonal after all. In tanks the seasons tend to add up to more than a 12 month year however.
So what to do to trigger them? This is an issues of ranges and trying the easiest. cheapest and least work things first.
Try using tap if its 6.8-8 pH. Know your starting GH. If its below about 5-6 degrees, you may need to raise it using additives. However, start by skipping weekly water changes to try and raise it. You can do minimal vacs and rinsing of media that is clogged and replace water removed- but try to minimize the amount of water that gets replaced. Raise the temp to 85-87 and feed the fish well on high protein foods, live is best, frozen works too. However, do mix some veggie matter in.
Skip wcs for anywhere from 2-4 weeks, though some have gone longer. Hopefully at some point after 2 weeks or so you will have a storm in the forecast. This can be rain or snow or any other barometric pressure changing event. When it hits, do a big water change, at least 50%. Now here is where I and most others part company. My experience has lead me to believe that the cooler water part is not necessary. However, it can not possibly hurt. So if you want to do it, drop the temp in the tank so its 5-8 degrees F lower. Disconnect the heater for anywhere from a few hours to overnight and then hook it back up. Also increase the flow in the tank- be sure you have flow directed across the mouths of the breeding caves.
If you are super lucky this may be enough. If it isnt you can also try doing a 25% wc every other day for a week or two.
The next line of defence requires ro or distilled water, a TDS meter and a way to control water params with additives. So try the easy way first
