I've got one about the size of a football. It seems happy- always open when the lights are on, and growing. If yours isn't happy, I'd check your parameters (alk, nitrates, calcium levels, ph levels through the day, temp, etc). If all of your parameters are in line, your ph and temp aren't spiking or dropping, and it's got medium light and medium flow, then I'd start looking at it really closely to make sure there aren't bugs or something on it that are irritating it.