artful

I think you need to be a bit more scientific with your water chemistry. Test then amend.

when using thread tape put no more than 10 turns of tape around the thread. Usually 8 will do, what ever size of fittings you are using. any more and you may split the fitting open. As for the tape getting into the filters and pumps, that will only happen if you allow the tape to cover the opening of the fitting so when you apply the tape just put it around the thread. As for the thread sealing paste, when you use it only apply from the middle of the thread up. As you screw on the fitting so the paste will squeeze into the thread. Only use a tiny amount. If you get it on your clothes it will not come off.

Cynobacteria is not necessarily caused by high nutrient levels. It can get in there in any number of ways. It is after all more closely related to a bacteria than an algae. Why don't you try dosing with vodka, it could do the trick. Try 1ml per 100ltrs for the first week then add another 1 ml per 100 ltrs a day. It's a persistant little blighter! Good luck. If all else fails you could resort to bleach. Remove the fish first though of course!

Hard to tell but from the anal fin it could be a Killifish. I'm not a breeder so probably best to try asking in the breeders forum.

the CO2 will lower your pH but you may need to stabalize the pH drop by using some pieces of coral in your filter. Measure the pH and GH and add or remove the coral until you get a constant reading. GH of 4 is a good level to aim for, also you could do 50% water changes weekly until the nitrates drop to an undetectable level, this will also drop the phosphate. you will need to bring the nitrates back up again as plants require nitrogen. for their green colouring, around 10ppm is a good level.