So I'm currently setting up my incubator. I had planned on using a submersible water heater in a few inches of water as I've heard that's a good easy way to make your own. But I'm noticing a lot of temperature fluctation. It usually goes between 87-89.5 degrees. It's a cheap heater I got off of eBay, is this much fluctuation ok, or should I invest in a better heater