Hi kev
I've been trying to cut through the discussion here to find all the revelant info to your problem, but if I miss something, please bring it to my attention.
I think you have a simple problem that is easy to correct. It will not cost much either.
Basically, your tank is dirty and you do not clean it aggressively enough. It is also overstocked which means that it will get dirtier than normal. Whatever filter you have, it will not clean up poop once it has settled on the bottom; that has to be siphoned out.
So.......If you can't fix your siphon, invest in a new one--it will be worth it to stop getting water all over the floor. You do not need anything fancy. Get a simple plastic hose with an attachment on one end that will give you an opening of about 1 1/2'. Don't waste money on any self starting models, because getting it started is a simple trick that you can learn to do with a little practice.
As Sinuhe explained, put the bucket as low as possible to take advantage of gravity. Then, start the siphon and plunge the big end as deep into the gravel as you can. Lift it out slowly and you will see it pulling the dirt out of the gravel. Repeat this as often as necessary, inserting the big end next to the spot you just did, to clean all the dirt from the gravel. Get as close to the plants as possible without cutting into the roots. Move your decorations (if any)to clean under them, but do not take them out of the water. If gravel comes out into your bucket, just put it back into the tank when you are finished.
Do not worry about removing too much water, or of sucking the bacteria out of the gravel. You can remove about 1/3 of your water at each cleaning without causing any problem. The bacteria clings to the gravel and is also in your filter, on the glass, plants and decorations, etc.
There is nothing you can do that will replace regular cleaning of the bottom, so get the habit of doing it and you will be pleased with the results.
BTW, do you know how to start a siphon without sucking on it?
