Well, you are right, you are in a "fish-in" cycle situation. Water changes will be your friend although you may grow sick of them before its all over. I'm catching on to your story now. You upgraded from a 5 gallon to a 25 gallon (in my numbers
) and I guess you are using the smaller tank for the fry? How did you stumble across this web forum?
The goal is to figure out, by trial and error, what pattern of water changes will keep the two poisons, ammonia and nitrite, down as close to zero as possible at all times. Usually the lowest test reading for these is 0.25ppm, so in practice you end up aiming to either see 0.25ppm or 0ppm on these two tests before you can rest easy and wait for morning or evening for the next test and/or water change.
Your two variables are the size of the water change and the frequency, of course. Your 30% last night sounds good but what always matters the most is that 0.25 maximum. If you've done a water change and are still above it, then you can do another water change after an hour (yes, you read that right, an hour!) In your case of seeing 0.50ppm for nitrite, it would be better to do a 50% water change.
The one thing we need to check on are your tap/source water test results: if you have elevated ammonia or nitrates in your tap water, that might cause us to recommend more frequent changes of smaller percentages.
What test kit are you getting? The strips, unfortunately, are not worth the paper they're printed on. You need to find a liquid reagent based test kit. Most of us here use the API Freshwater Master Test Kit or the Hagen/Nutrafin Mini-Master Kit (the one that has the nitrate test.. some don't.) Once you get your kit, its great practice to learn the tests by doing a few on your tap and tank water and posting up the results here in your help thread.
The thing that's exhausting about a fish-in cycle is that the water changes keep being needed day after day. It usually takes about 4 weeks but in truth is quite unpredictable. Eventually, when you can go two days with zeros for your results, you can breath easier and begin a week of double-check watching of the values, after which you can declare yourself cycled.
You are not over-stocked, which is one good thing on your side. You may be roughly at about 18" of fish and 23" would be a good max for you as a beginning community tank. Of course, no more fish during cycling! Your SAE may grow to 4-6" max (I think) which will be quite large for a 25G/96L but should be ok if you don't overstock on other things.
In your fry tank you'll need to do more like 90% daily water changes for a short period I believe. Perhaps you can check in the livebearer section or a mollie person will happen along. You probably need to make the flake food really pulverized and you may need other special foods, not sure.
When you sit down to rest from all your water changes, log on here and read the pinned topics about the nitrogen cycle. There are some helpful charts on wikipedia and here and there on TFF. Its a great thing to ask questions on.
OK, that's all I can think of for now, good luck,
~~waterdrop~~
)
