I am in tropic zone, so mostly the temperature is between 25-30 degrees celcius nowadays. I dont think the temperature drops quickly at my place. Could you suggest me causes of sudden temperature drops ?If it is a condition, there has to be a cause, do check the pH and other parameters, though you said it has been set up for years and you change 30% every week? Is it even possible for the temperature to drop quickly ? Which temperature is it? Do post more and better pics, the pics you posted aren't bad, but like I said not good enough to see what is going on.
for your reference I have attached few more pics. Hope thats helpful.