A 75% water change is fine to do as long as the new water has a similar GH, pH and temperature to the tank, and as long as it is free of chlorine/ chloramine before it is added to the tank.
Big water changes dilute nutrients and disease organisms more effectively than smaller water changes.
If you do a 25% water change each week you leave behind 75% of the bad stuff in the water.
If you do a 50% water change each week you leave behind 50% of the bad stuff in the water.
If you do a 75% water change each week you leave behind 25% of the bad stuff in the water.
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The fish looks like it is gulping air from the surface. This is unusual but could be caused by low oxygen levels in the water. Increasing aeration/ surface turbulence might help, as could a big water change.
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The gravel should be cleaned every week when you do a water change. The easiest way to do this is with a basic model gravel cleaner like the one in the following link.
Aquarium cleaning should take 30 minutes every two weeks. Learn how the professionals do it in 18 easy steps.
www.about-goldfish.com
If you don't have or can't get a gravel cleaner, you can make one from a 1, 1.5 or 2 Litre plastic drink bottle.
Cut the bottom off the bottle and throw the bottom piece away.
Remove the lid and plastic ring at the top of the bottle and throw those 2 bits away.
Put a garden hose in the top of the bottle and run the hose out the door onto the lawn, or cut a length of garden hose and put one end in a bucket.
Use the bottle as the gravel cleaner when you do water changes.
The fish stay in the tank when you gravel clean the substrate.
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How do you clean the filter?
