Just trying to get some opinions on this...when any of you do a PWC, how long do you wait to test the water for ph, amonia, nitrates, nitrites, ect.? 1hour, 8 hours, 24 hours...? Also, in a 75 gallon tank, approx. how much gravel do you think should be vacuumed?
I'd probably wait a few hours before testing the water after a water change. In my 100 gal. tank, I vacuum all the gravel on the bottom, but then I have a lot of filtration on the tank and it's got 2 oscars and a common pleco in it, so I guess the answer to your question is if you have a lot of filtration, vacuum it all if you want to. If you do that, and your water gets cloudy after doing maintenance, then you probably need to bump up the filtration, or vacuum less of the gravel each time. A good rule of thumb is to vacuum no more than half of the tank so your bacteria isn't too disturbed, but like in my case, my fish are huge waste producers, and I have extra filtration on the tank, so it's not an issue.
You might want to bump up your filtration with another Penguin 350. Ideally, you want your gallons per hour to be 10 times the size of your tank when using HOB or sponge filters. Another Penguin 350 would almost get you there.