90G unattended Co2 or excel?

In that situation, if it was me, I would go low-tech. High light like that can cause problems to occur really fast if something goes awry. I would be afraid to leave a tank for a week with lighting like that, much less a month, and that with no water changes. A non-co2 or Excel tank with low light (maybe half what you have there) and a rich substrate will be much more forgiving. A month is a long time for any tank to go without water changes, much less a planted tank and high-tech at that. My .02 anyway. Non-co2 tanks can be very nice, without nearly as much hassle. The lighting is the issue here I think, that determines the speed at which things take place.

 

I agree with most of the above, I think CO2 is out of the question. You could use excel when your around. Also turning the lights down to a reasonable amount of hours while your gone should help.

I'm not sure that that light would be too much? but maybe you coulds only run 2 bulbs while your gone?

 

Effectively you will have over 3 watts per gallon on that tank, far too much for an Excel only tank, and equally far too much to leave the tank unattended for 30 day periods. You would almost certainly return home to an algae filled mess the first time you left it. It is work and it can be tricky to use even less light than that, observing the tank every day, cleaning it weekly, doing water changes weekly, and hoping to get a good result. I agree with Carissa that you would be much happier with a no-CO2, low light tank, where everything happens over a much longer time span. Even then, 30 days is a long time to leave the tank unattended, except by an uninformed, un-motivated "helper".

 

I have to agree with the other replies, that a low tech, low-light setup would be better for you.

With high lighting and c02, you need to fertilize. With ferts, you need water changes.

I would go to 1 or 1.5 watts per gallon. See if you could mount the lights so that you can raise them a few inches when you are away. That will reduce the growth and nutrient requirements and make it a little easier.

 

Because you have T5 lights, I assume with individual reflectors for each bulb, you have at least 1.3 times more light per watt than the watts per gallon "rule" is based on. Possibly more than 1.5 times more light. So, the 2.4 watts per gallon is effectively at least 3.2 effective watts per gallon and possibly more than 3.6 effective watts per gallon. The depth of your 90 gallon tank is about what is normal for that size tank.

If you have someone looking after the tank, who is well versed in how to care for a high light tank, and well motivated to do the work involved, then you aren't really leaving the tank unattended for a month. But, from your description your "helper" doesn't know how to care for a high lighted tank, and, as a result I doubt that she would understand or be very sympathetic to the need for routine maintenance, pruning, cleaning, etc. that such a tank has to have. Undoubtably you could make this work, but it will be much more likely to succeed with lower light.

 

"Low Tech" means low light intensity. You can't successfully reduce the plant growth rate by restricting fertilizers. You can only do that by restricting the light intensity. If you use only one of those two light fixtures you will have a chance to succeed. To do that I suggest reading about Diana Walstad's "el natural" method for running a planted tank. One place to do that is http://www.aquaticplantcentral.com/forumapc/el-natural/.

 

^ 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, 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....

 

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.

 

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.