Plants And Stocking Levels

You'll probably find many answers to this, some vehemently opposed to others!

My tanks (9 at present) are all well planted and are all overstocked according to the community creator. I've had no problems whatsoever with fish health or water quality.

Plants are undoubtedly capable of using up fish waste products. After all, people kept fish successfully for decades before the modern concept of biological filtration came in, mainly by aiming for a balanced tank in which plants and fish complimented each other. In theory, an adequately filtered tank which is also well planted can hold a higher stocking level than one with few or no plants. Having started with a soil under gravel substrate, my plants were growing pretty well before I added any fish, but there was a real spurt in growth once the fish went in. I used ammonia for cycling, which would have provided plenty of nitrogen, and the soil would have enough of the other nutrients. This leads me to conclude that it was the extra carbon dioxide from the fish that was the main reason for the spurt in plant growth.

Unlike Diana Walstad (I don't always agree with her), who says that biological filtration is not necessary in a densely planted tank, mine are actually over filtered, in most cases by having two smallish filters rather than one oversized one. This gives good filtration without causing too high currents and would be less disastrous if one failed.

I do try to create a mini ecosystem in each tank. As well as plants, I also have snails in every tank plus some other invertebrates which I'll get round to writing about some day. My tanks are not neat underwater  "gardens", but a tangle of plants including Java moss and lots of floating Amazon Frogbit with long roots trailing down and, yes, filamentous algae in some cases. My aim is to make something that looks like a natural environment rather than a pretty picture and I believe the fish, which are all fairly small and in shoals, are comfortable in these conditions. Many people will probably scream in horror at my stocking densities, low level water changes, low temperatures (23^oC or less and other things, but "the proof of the pudding .................".

I have digressed quite a lot, but I would say that you definitely can stock more densely in a well-planted tank and that your 75% is well below what would be comfortable. I'm sure others will have a contrary view.

Plants do remove heavy metals, though that shouldn't be an issue with an aquarium as I guess everybody uses a conditioner that deals with them. Perhaps the other main thing is phosphate, which can be high in tap water and is usually blamed for algal blooms in rivers and lakes. Aquarists worry about nitrates since they have an effect on fish but tend to ignore phosphates completely. Algae in tanks is usually blamed on too much nitrate or too much light, but I often wonder if phosphates are a factor.

According to Walstad, breakdown of organic matter is one of the main sources of CO₂ for plants in aquaria. From my long ago organic chemistry, I seem to remember that CO₂  and H₂O are produced in many reactions, most equations could be balanced by a molecule or a few of these! Certainly, I virtually never get any accumulation of mulm in my tanks, which I assume is because I have a good population of heterotrophic bacteria which break down the fish poo etc into smaller molecules which the plants use. The snails probably help in this process. Regular weekly cleaning of the substrate probably greatly reduces the population of bacteria.

To quote Walstad again:
"Having a ‘dirty’ aquarium that will support good plant growth requires a leap of faith. The hobbyist must believe that plants can help purify the aquarium".

Personally I would change "believe" to "be convinced by the evidence" since I don't believe in belief  .

I've no idea. Personally, I wonder if these are really a common issue, certainly I've seen no evidence myself. Though I would imagine that a biologically diverse tank with a wide range of bacteria would be more likely to break down any organic compounds present than would a super clean and relatively sterile one. Maybe that's where these problems occur??

I broke the rules (again) by putting all of my fish in at once in most of my tanks. It made sense after spending all that time adding ammonia to build up and maintain the bacteria levels, and I waited weeks (actually months) for the whole tank to become mature, rather than just the filter. I've had no problems. Adding new fish to an established tank always brings the risk of disease, but getting them all in at once from one source meant that disease would likely appear pretty quickly and at least I only had to look out for it for a couple of weeks rather than every time I added new fish. And of course the fish I added were by no means fully grown so that I was not actually stocking fully, while the filter bacteria had time (if they really needed it) to grow with the fish.

It would be interesting to see a proper reference to the phosphate/nitrate balance for algae, I'm not quite convinced until I do!