Hi Richard
I know exactly how you feel, I'm also suffering from exactly the same brown smudgy diatom mess as you, during the last 2-3 weeks it just appeared, I'm from Rochester so not that far from you. I find that strange as I presume your with south east water as well. Anyway I don't think our problem is due to our water supply as diatoms are mainly because of to much light or a immature tank I think. However I don't think my diatoms are from either of them issues so its odd.
I've received great help from this forum and there are also many threads about diatoms on here which have also helped me. I'm still battling the brown algae, I have tried a 4 day black out, it helped but didn't kill it all. I then noticed it was still spreading on my glass/plants. I then did a soft black out which is just turning the lights out. That helped also, along with many water changes. Around 70% a week. I tried to increase flow all over my tank. I changed my diffuser location, I changed my whole EI dosing from a starter guide which was dosing 10ml every 50 litre, and I now have measured and dose all my powders dry. I now know exactly what I dose rather than using a guide.
I tried to rub as much brown algae of the leafs as I could but with carpeting plants this was nearly impossible, my plants were brown, they looked dead, they were rotting, leafs were falling off, the tank looked dead. I was going to remove some of my dead plants at its worst point, but I left them in hope. After doing all this, a few days later I noticed green patches on all my plants leafs, no not green spot algae, but the brown algae was receding. I could see green again I couldn't believe it especially with my echinodorus! I then started to see new growth, green tips from all my plants, each day its going bit by bit, I've got a long way to go. But my plants are fighting back, and my stauro repens and glosso, which all looked dead, are not putting out tiny green shots at the substrate.
So don't give up, I felt really depressed with my tank, I've only ever had 2 aquariums and my first was about 2 years ago which was just fish and plants, I new nothing about this hobby, who ever looks at a fish tank in a shop and would guess there is a site just dedicated to the stuff that is going on inside it which you can't see , and I've learnt more on this forum in the last 2 weeks, that any other website or information I found on Google. So don't give up. I think it just takes patients, as mentioned above it don't happen over night. My tank is so embarrassing compared to tanks on here. But I didn't give up and my plants are now fighting back.
In regards to your fish, mine would do the same, I would walk in my room sit down and my tank would be empty, even though I have like over 22 fish in there, I would then notice all the fish hiding at the bottom in a corner breathing rapidly. I didn't understand back then, I did so much research I was told it could be stray voltage, I wasted money going down that road and to this day I still don't know how to find out about stray voltage, but when I turned CO2 off for a few days they all came out more and the weird rapid breathing went away. I never once thought it was CO2 as I've seen fish gasping before at the surface etc, and mine was doing it at the bottom which was odd.
I now keep my CO2 low and surface agitation up, and it seems to help, however with my 200 litre tank I've never achieved a green drop checker, when I try turn my co2 up, the fish mostly in the last hour or two of the photoperiod start acting odd, rapid breathing, hiding, eradicate swimming, so I can never get a green drop checker even though I know its only a guide to co2 levels, but I find it hard with in tank diffusers to dissolve all the bubbles.
It was only yesterday I turned it up from 1.2bps to 1.5bps and my fish acted strange again in the last 1-2 hours of the photoperiod, I tried to up my CO2 slow so they could adapt as I read it can take 24 hours for fish to adapt, but nope.
I know my tank is only 200 litres but I have 3 small clown loaches, yesterday for the very first time I even witnessed one rise to the surface for a gulp of air, and I could see he didn't look right as he look dazed out, he managed to get to the surface though. Today for the first time since I've purchased them, all 3 never came out of my ornament, I started to think the worst, but as soon as lights went out they came out thankfully, and today I have lowered CO2, its to stressful for me I just cant get CO2 right in my tank.
I'm actually thinking of just giving up with pressurised co2 and trying carbon instead. Imo I dissolve all my CO2 quite good, blue drop checker, still gassing fish. Sometimes there are no answer.
Anyway my fish have to come first.
I could go on all night about my problems and what problems I've had, its not easy growing plants, especially for me. But I wouldn't give up, because when it does go right, its such a joy to sit back, admire what you've done, and watch the fish show there natural behaviour. That to me is worth carrying on, and its more of an achievement when you do get there in the end. I hope you can see your not alone, and I know your plants are more carpeting plants, but I'm sure they will bounce back, even when you think there dead! Hopefully things can only get better for you now. Sorry to go on and on, but I don't think you should give up, give it a month or two
Good luck! I hope things do work out for you, as its not nice.
