Saving Gregory the Oscar’s tank

[Marineboy22]

Hi all, this is my first post on this forum. I have recently Acquired a 125 gallon freshwater tank with an oscar and a very small beige colored suckerfish, clearly not a common pleco. To say things simply I started knowing basically nothing about fish tanks and I am learning slowly. I bought a 5 panel test kit that gave me this info:
Ph: 7.5
Alkalinity: 720ppm
Hardness: around 900ppm
Nitrite 0ppm
Nitrate: 50ppm
The tank was neglected for a while, the oscar wasnt fed for months by my old roomate and clearly stunted by it. He has been here for roughly a year and only about 3-4 inches and he was not much smaller when my roommate got him. Yesterday I decided to start trying to give Gregory the Oscar a better life. The tank has quite the buildup of algae and the filter was caked so i cleaned the filter sponges and racks and took the driftwood out of the tank that was coated with algae and gave it a good pressure washing. As far as water parameters go I know I need to do something about the General Hardness and Alkalinity and was thinking about using RO water from the aquarium store for my first water change. Last water change was a month or more ago done by my ex roommate with well water. (Probably why the gH is so high). Next step of my rescue is probably going to be a water change and getting a gravel vacuum so I can vacuum up all the nasty algae and poop in the gravel. Gregory seems okay except from his weird life he has endured he is quite skittish and seems to have developed a nocturnal pattern being the most active whenever the lights are out. (Ive even gottem him to take food from my hand in the dark) and he seems to be in rough shape as far as his fins go. They dont look the best. I was worried about his white coloring at first as well until I read around some and realized that he may just be a Zebra Oscar and the white coloring may be normal for him.
Attached are some close up pics of Gregory with his tank mate the sucker right above him. Also pics of the tank after doing my cleaning yesterday. Would appreciate and tips/pointers from seasoned fish keepers. I want to give Gregory the best life possible.

 

[BigManAquatics]

Changing the gh can cause more damage. I would keep him on the same water ande see how he does when someone takes an interest in his care.

 

[StarGirl]

I agree dont change everything too much too soon or it will be more harmful than good on your part. Baby steps.

 

[SamMe]

This is a great thing you're doing for Gregory. Did you happen to clean the filter sponges in old tank water? My only concern about your cleaning project is that you make sure to preserve enough good bacteria so you don't mess up the tank's nitrogen cycle.

BTW from the looks of the tail, Gregory's friend is an albino bristle nose pleco. Great little cleaners that stay relatively small.

 

[Marineboy22]

Thanks guys for the answers, I purchased a gravel vacuum yesterday, if and when I vacuum the gravel i will make sure not to remove too much more than the layer of algae on top since im sure the gravel holds tons of beneficial bacteria, the back wall is still coated and i have algae wafers in case the sucker gets hungry. I will need water to put back in after my vacuuming. You guys are suggesting I should use water with the same parameters as whats in there already? The guy at the aquarium store sold me some “prime” which he told me is good for conditioning tap water to remove chlorine and ammonia for water changes. The only thing is my tapwater is relatively soft so it would most definitely lower the Hardness some correct? These are my tapwater parameters

Ph:7.0
Alk:180
Hardness:90 ppm
Nitrite:0.5ppm
Nitrate:20ppm

Also before I cleaned the filter I did some reading about tap water vs tank water for cleaning and decided that i would use tap for the rinsing since i had no conditioned water to put back in after cleaning. I did not touch the filter media except for giving it a light rinse in the tank water that remained in the filter just to get the heavy gunk off. Since this is a well established tank I suspect that theres millions of bb in the water, substrate, and in the media so I dont think there will be a problem, although I could be wrong.