Hello Sarah! Welcome to the forum - and fishkeeping!
Sorry that you've been having such a difficult time. . . lets see if we can figure out where the problem is. . .
There are a couple of things that I'd like to know about the process you go through during your water changes...
First, I'm not familiar with the product called Fresh Start, but you must be sure to add something into the water that will remove all traces of Chlorine, Chloramine, and heavy metal toxins before you put it into the tank. Is this what Fresh Start does?
Second, are you making sure that the temperature of the water going in is the same as what is in the tank already? It's very important to do so, as a rapid change in water temperature can prove fatal for the fish.
As a general rule, water changes should be done every week - not every other or once a month. Are you doing them once a week, and only having problems during some water changes?
You mentioned keeping your Ph at the correct range with the use of Fresh Start. I've found that using chemicals to keep Ph within a desired range is seldom the best way to go about things. What tends to happen quite often with this method is that the Ph will shift suddenly, usually when the chemicals are being added, and this is very stressful on the fish, and deadly. . . there are ways to safely harden or soften your water, but the best way to stock a fish tank is to choose fish that are ideally suited to your tap water in the first place. It's easier, cheaper, and all-around the safest way for everyone involved!
You mentioned testing your water, but you only specifically speak of the Ph and Nitrate levels. We usually look at Ammonia and Nitrite first, as any trace of these in the water are very dangerous to the fish... oddly enough, to fish that have become used to living with trace amounts of these toxins, the sudden removal of them from the water can also stress their systems enough to cause harm. If you don't have one already, I strongly recommend that you purchase an accurate liquid
testing kit (with vials and chemicals), and check out the levels for ammonia and nitrite as soon as possible. I'm not entirely sure where in this world you are located, but here in the States the Freshwater Master Testkit by API is the most highly recommended one.
Here is a link on the sticky on this site called A Beginner's Guide to the Freshwater Aquarium Cycle
. If you aren't very familiar with the nitrogen cycle, I HIGHLY recommend you give it a read! The cycle is crucial in keeping healthy fish in a closed system. It can be a bit mind boggling at first, so please feel free to post any questions you might have!
Because of the bacteria that is necessary to have in your tank, it is not recommended that you should ever remove the gravel and clean it, as this beneficial and necessary bacteria lives on the surfaces inside of your tank - the gravel being a main one. If you clear away the bacteria, you can cause the toxins in your tank to spike, which. . . you guessed it! Can harm the fish. . . this is also why the PP is concerned about how you're cleaning out the filter. If/when you must clean the filter, you are correct in that it should be gently done (so as not to disturb the bacteria) but it also must be done in dirty tank water for the same reason.
As far as algae is concerned, usually algae becomes a problem if your lights are kept on for too long, but also if there is a high phosphate level in the tank. Phosphates, essentially, are wastes. If you aren't doing a water change every week, this will certainly contribute to the algae problem, as it gives it a source of nutrition that will allow it to thrive. I'd start by cutting back on the amount of time that the lights are left on, and we'll go from there.
This probably sounds pretty overwhelming!!! I'm sorry for that, but don't worry! There are lots of experienced fish keepers on this site who will be happy to help you set things right for your fish's health! Many of us (including me!!!) started off on the wrong foot, it's a lot to learn, but worth it for a tankful of happy, healthy, thriving fish!