Next, using a gravel vacuum, clean the gravel to get out as much junk as you can. It looks like your gravel is pretty deep and uses a large grain size. For that reason you may have to stir up the gravel and then replace 50% of the water. I'd only to this is the gravel vac doesn't appear to be lifting out the dirt from the gravel. If you do stir up the gravel and remove 50% of the water, the tank may appear dirty for a day or so. You may even have to repeat the gravel vac in a week or so but you should be vacuuming it every other week anyway
If after all this you have to look for two things: 1) Are the bubbles flowing strong enough 2) are the bubbles flowing evenly between the four tubes. If the bubbles aren't strong enough you’re due for a new pump. If the bubbles aren't even then you'll need to buy a gang value to control the air flow to the individual tubes.
Here's what I do and I have the same setup as you although I'm only using 2 of the four lift tubes.
On each lift tube you should be able to remove the top leaving the long tube connected to the undergravel plates. You can buy a brush that will fit in the long tubes to clean them without removing them from the tank. When you remove the tops, a long skinny tube should come out with them. At the end of these long skinny tubes is the air stone that can be replaced. The LFS store should sell them. They come in fine or course bubbles. I like the course ones. You can put the tops back on the long tubes after you've replaced the air stones and cleaned the tubes
i like the way u put it all so im understanding this right i need four stones i need to take the gravel out clean the plastic grates i have underneath it and put it all back together is that correct and then it should bubble better and i think i have that thing ur talking about i got one thos green things with 4 air lines that come out of it is that wut ur talkig about