I've found that using cultured phyto, versus the paste / frozen stuff, gives me a more dense concentration of rotifers and, a lot less "crap" in the rotifer bucket, (what ever is not eaten, just multiplies, so, it's just more food for the rotifers
). Culturing is EXTREMELY simple, and, requires next to no time to maintain, (once a week, or so, depending on how long it takes to go dark green, I split the bottle into two bottles, top it up, and add about 2 mls of Miracle Grow Liquid, and put it back under the lights... that's it... well, along with providing light, (I use a 13 watt compact fluorescent, screw in bulb that's on 24 hours a day, along with an airling in each bottle). I've got seven two litre bottles right now, and have yet to have a crash. Just make sure you are careful about cross contamination, (all you need is one rotifer in your phyto to turn your phyto culture into another rotifer culter
). As for freezing... I'm not sure what will happen to it. I culture mine indoors, so, it might be a good idea to do the same, (unless it's really warm, or at least about, say, 5 - 7 C, in which case, you can do it outside, I guess). I hope that helps.
Robert