That's exactly what I have been saying. Because of the possible "danger" of water changes in low light, slow growth tanks, "EI" as the term is usually used (ie, large, frequent water changes with additional nutrients without too much concern about ambient nutrient levels because the next water change in a few days will reset those levels) cannot be used, because of the chance of nutrient buildup during those few and far between water changes.

In my tanks of that type I add NO3 and PO4 if required, when I top off the tanks. Don't ask how I know they are needed. <g> I also dose traces weekly, as of now.

BTW, in addition to CO2, new water can also add other nutrients, like nitrates and phosphates, presenting other problems in low light tanks.

Bill

Adding a bit too much nitrate and phosphate with water changes does not, as far as I have been able to find out, cause any other problems in either low or high light tanks. Before either would be a problem the tap water you use would be out of the limits for drinking water.

True, if they are added in more or less appropriate ratios. But if there is all nitrates or all phosphates, for example, problems could develop, couldn't they?

I'm going to start a new thread with that one.

Bill

Thank you all for your replies.
I have decided (after reading your posts) to go with Tom's recommendation by fertilizing 1/3 of usual EI dose for a 90 gal. and will do a %25 water change this weekend. and we will see what happens ,I'll keep in touch in 2 weeks with the results.
Thanks again for all your input.
Ken.

The notion here is that you may use the math of EI to tailor any set dosing routine/water change method you wish to get a range of NO3, PO4 etc.

EI is and never was written in stone as a 50% weekly water change.
You may do more, less, skip a week water change here and there etc, do more than once a week, less, larger than 50%, or less.

I prefer to to do 70-80% personally, once a week or sometimes more often when testing.

Some folks do daily water changes with automated solenoid based water changers. They still dose 3x a week etc, they just don't worry about water changes, just dosing and CO2, pruning and feeding of fish.

You know.....the fun stuff.

Low light tanks means that the maximum assumed uptake is much lower, say about 1/2 for 1/2 the light, while not absolute, that's not a bad rough guide.

Non CO2 tanks that get no water changes can still be dosed, generally once a week and only just enough and that seems to allivate the limitation issues.

I dose based on what I know of plant uptake, less so for fish waste contributions.

I spent 3 full days with Ole and Troels the "bottomless pit" (Tropica and both spoke at the AGA event) and we agree on most everything.

It's the hobbyists that have the issues, we already know the info with dosing/plant uptake/ratios/growth rates and what determines all that.

Regards,
Tom Barr

Hi all
Just to update,I have been dosing EI but half the dosage for 90gal.ever since I dosed NPK+trace,I also changed the lights to Coralife 4X28W T5 normal output.the plants are doing amazingly well,I have next to no algae,co2 is 30ppm.
the bottom line is adding co2 was the best thing for the plants.and T5s are very efficient .It doesn't look like I need more light. ,