^ Further in explanation to Vaughn's comment above... the use of Excel or CO2 is not actually what drives the growth. It's the lighting that drives growth. Adding a source of carbon and other fertilizers is just something that you HAVE to do if you plan on driving your growth fast with high lighting. Slacking off on fertilizing (including carbon which is one type of fertilizer) while continuing to drive fast growth with the high lighting will quickly result in algae, deficiencies, and ultimately, die off. Think of the amount of light as the speed at which you are driving your car, and the fertilizers (carbon included) as the gas that you're putting in your tank. Putting less gas in your tank will not reduce the speed of your car, it will just get you into trouble if you run out. The only way to reduce the amount of gas you burn, is to drive less. Doing water changes is like doing the oil changes on your car. The more miles you put on it within a certain period of time, the more often and more crucial it is to do oil changes to keep it in good shape.

Personally I use the dry ferts, they're cheaper. I don't see any advantage to buying the Seachem line, it's doing basically the same thing.

Yes, I understand what you guys are saying, but I think I was misunderstood on my last post. I did mention reducing the lighting intensity aswell.
Say you had 2 w/g and u used ferts 2 times a week. If you cut your lighting down to say 1-1.5 w/g and added fertilizer only once a week would that not just slow down the growth of the plants as long as there was the right balance?
If its the light and fertilizer that most important could you not kind of play around with the lighting levels and fertilizer levels to achieve different growth rates? Kind of like how plants might go through different seasons in the wild at different times of the year depending on whats available to them at that time?

Or am I completly off the wall on this?

Yes, you are right, reducing lighting reduces the need for fertilizers.

However, I would be a bit apprehensive about doing this. Plants thrive in a stable environment (especially when it comes to carbon). Suddenly decreasing the amount of carbon and light could still result in problems. Plants adapt to their circumstances to make the best use of what they have available, change is almost always a bad thing for plants and takes time to adapt to it. I don't think you would gain much by keeping your lighting high, if it means you have to start messing with it later on. If you are going to do this, I would test it a couple of times before you actually go away. Any change in conditions can cause problems so beware.

If you did decide to do this, I would definitely not reduce your carbon even if you lower lighting and other ferts. Reason being that plants adapt to the amount of carbon available, and if you go from high carbon to lower carbon, plants basically stop growing completely as they adapt, which can take a week to a month to do. During that time algae can easily start taking over, as the plants are suddenly at a standstill in growth with their handicap.

Another issue you have is that not all plants will do well under lower lighting. So plants that appear healthy, if you were to suddenly reduce lighting, could die off. Plant die off = algae, algae = more plant die off, and it just snowballs.

It's a very risky idea. I'm not saying it can't work, but in practice I would be surprised if you didn't have some problems.

A better idea would be to get your gf into planted tanks....

Alright, So I am going to try and find a stable balance of light (not the full 4 x54, as you say it is too much without co2) , fertilizer and excel that will work and can go a month with out water changes. I can use a doser for the excel and add ferts 1-2 times a week. I think this should be possible, but I guess I will have to play around with it and see.

Any recommendations for plant load? I know heavier is better, but whats heavy...100 plants in a 90 to start?

I guess it depends on the size of the plant....have a look at some pictures of fully planted tanks and keep buying plants until it looks right. Some fast growing stem plants are good to start off with. Think more about covering your substrate than looking at the height to start off with, the plants will grow taller to fill in the height but they are less likely to grow out to cover the substrate (depending on the plant naturally). If you spread out bunches of stem plants, they will grow faster than if you leave them bunched up. You can always bunch them up later on. For a lower lighting tank, I've had good success with java fern, crypts, and some hygrophila polysperma as a faster growing tall stem plant. Anubias are great too. Easy care, low fertilization requirements (except the hygro, which will suck up fertilizers faster since it also grows faster than the others). I'm actually happier with my tank since I changed to non-co2. It seems like before, I could never keep up with the pruning, and algae was always an issue since I was doing diy co2. Now, I have nearly 0 algae, the tank looks great all the time, not just after pruning. Plus I don't seem to have any issues going 2 weeks without a water change. You also want lots of algae eating fish, cuts down the maintenance considerably. I have 4 oto's in my 30g.

Heavy planting means a plant every square inch of substrate, if not more. So, if that 90 gallon tank has a substrate area of 900 square inches, heavy planting means planting 900 individual plants. That seems overwhelming, but if one of the plants is Glosso, for example, a single pot could have 100 plantlets in it, since you can plant cuttings that have only two leaves. And, a bunch of Rotala green could have 20 or more plants, since you can cut the stems in half and plant each half. Using tweezers to plant, planting with very little water in the tank, you soon get a rythm going and it doesn't take nearly as long as you might think.

Well, thanks for all the help and I am going to start looking into some good plants to use!

Any ideas as to whats the best groundcover plants for low light?

Marsilea minuta (PlantGeek.net - Marsilea minuta) is the best low light ground cover plant as far as I know. I had some problems getting it to start growing, but then it took off and made a big mass of ground cover. I did have high light at the time though.