The tank is 4-5 months old. Recently algae did grow on the side for the first time, but it was pretty gradual - I don't really think it would count as a bloom. I did add more carbon to the tank filter b/c I was worried about some weatherization work being done in the house and the chemicals getting in the tank. That work wrapped up about a week ago and I was planning to leave it in til this weekend even though I don't think that the chemicals from the weatherization work ended up being that bad. The only other thing that has changed is that 4 new baby fish were born - but I don't think the tank is overloaded. I have 5 full grown platys, 4 plattys that are about 2 months old, and 4 1 week old plattys. I intend to find a home for the 4 2 month old plattys shortly.
Other than that I am at a loss for what has changed. Around 4 months ago is when I switched to RO water, and I keep the nitrates/ammonia down that way - generally I don't let the nitrate go about 20 ppm. However it is at 20 ppm now and I am hesitant to do a water change b/c my kh is now down to 1.
When I first started with the RO water, I was waiting for crushed coral to get here in the mail (my nearest lfs is 45 min away); and I used small amounts of baking soda instead -- but ended up losing a fish to ph swings so I am very hesitant to go that route.
Any other suggestions?