I do think a 25% weekly water change might be a bit excessive.
Normally after the cycling process is finished (~4 weeks), we suggest a 10% weekly OR 20% every other week water change, with average bio-load. Ultimately your parameters (esp. nitrate) will dictate whether more or less is required.
Changing so much, so often, so early in the process may be causing the tank to lose equilibrium. It finds equilibrium when it cycles. Changing too much water, too often, effectively hits the "reset" button and it tries to find equilibrium again.
That might be why you're having cyano issues. Are you having lots of diatoms too?
I don't suggest the other extreme... lots of people "get away" without water changing very often, but their creatures would do so much better if they had a regular replenishment of trace elements through water changes. Also, if you aren't changing water and aren't testing because things look fine, that's often a time bomb... I can't tell you how many people have had things on auto-pilot only to hit the wall with nitrates, and once they start having problems, the nitrate has gone off the map and it takes a long time and a lot of small, but frequent water changes to put things right again. An ounce of prevention really is worth a pound of cure.
Keep an eye on your parameters, and ease up on the water changes a bit, IMO. 10% per week or even every other week if your bioload is light, should suffice for now, and give the tank a chance to find its equilibrium. Test for CA/ALK/MG and dose as needed, but until the tank settles some more, don't stress about chasing a number quite so much.
I've seen people drive themselves crazy chasing numbers... and if they don't hit their numbers without throwing something else out of whack, sometimes I suggest just backing off of everything except feeding and buffering pH (if it's needed) for a while to let the tank settle. More often than not, levels settle closer to what the hobbyist was trying to achieve. Once it's settled, then it can be tweaked here and there without excessive measures. We really can tinker things right to death.
Jenn
