The mulm isn't really a bad thing all the time.
Here's my suggestion:
- Take out the filter, and put the media into an old bucket or something with a bit of tank water.
- Rinse out the filter completely. Remove it as best you can. You can use tap water for cleaning the FILTER but NOT THE MEDIA!!!
- Give the media a gentle rinsing IN THE OLD TANK WATER, and replace it.
- Put the filter back in the tank and fill as normal. Turn it on and all should be fine.
Dealing with Old Tank Syndrome is another thing altogether.
What's your water change regimen?
What's your nitrate reading?
If it is Old Tank Syndrome, that's easily fixed too, but you will need to do some extra water changes to make up for the ones you haven't been doing in the past.
Do a daily 10% water change for a week.
Then bump it up to a daily 20% water change for a week.
Then you can just do biweekly 30% water changes for a while.
After 2 or 3 months, your fish will be ready for a 'big change' and you can do a 50-75% change.
The key is that the fish's bodies will need time to acclimate to the new (cleaner) water before trying to acclimate them to even newer (and even cleaner) water. This takes time. They got acclimated to the current water over a long period of time, and need a long period of time (not as long as it took to get it this way) to get used to the new water.
Imagine your nitrates were at 200ppm (a rather conservative value for 'old tank syndrome') and daily your nitrates are rising by 5ppm (which is also rather conservative for a fully stocked tank without plants) - ***assuming tap water has 0ppm nitrates.
A 10% change would only lower it to 180, but would rise back to 185 at the end of the day.
The next day it would be lowered to 167, but rise back to 172 at the end of the day.
Day 3: lowered to 150, but rise to 155.
Day 4: lowered to 140, but rise to 145.
Day 5: lowered to 130, but rise to 135.
Day 6: lowered to 122, but rise to 127.
Day 7: lowered to 115, but rise to 120.
This first week will lower the nitrates (and whatever else is in the tank) slowly giving the fish a chance to acclimate to the water.
Whereas just doing a 20% change at the end of the week would only lower the nitrates to 160, but they would rise back to 195 by the end of the week. So, the fish would be experiencing a large nitrate (and other stuff) shift at once, and they would be back in basically the same water as they started in by the end of the week.
During the second week, the larger percentages of water changes would bring the levels back quicker to where you want them...
Day 1: lowered to 95, but rise to 100.
Day 2: lowered to 80, but rise to 85.
Day 3: lowered to 68, but rise to 73.
Day 4: lowered to 50, but rise to 55.
Day 5: lowered to 45, but rise to 50.
Day 6: lowered to 40, but rise to 45.
Day 7: lowered to 36, but rise to 41.
Now the levels are about where they are supposed to be (assuming ONLY an initial nitrate level of 200... it could be much higher).
Week 3: Wait 3 days to change the water: nitrates have risen to 56.
First water change of the week: Lowered to 40, but rises back to 60 by the end of the week.
Second water change of the week: Lowered to 42.
Week 4-5. Maintaining this level for about another 2 weeks will allow the fish's bodies to acclimate fully to this new water (it can take a week or two to acclimate FULLY to new water chemistry changes).
Week 6. 50-75% water change brings you very close to where you need to be.
One water change this week of 50% lowers nitrates to about 30, and will rise back to 65.
One water change this week of 75% lowers nitrates to about 15, and will rise back to 50.
Week 7. A final water change of 90% should finish the process and bring the tank to where you want the nitrates, etc. to be.
Water change lowers nitrates to ~5, and then rises to 40 at the end of the week.
Another thing to note: the nitrate produced each week SHOULD decrease as you do all these water changes, assuming you are sucking up the poo, etc. that is in the substrate during each water change, and not just changing the water. Only concentrate on getting the obvious detritus in the bottom of the tank out in the beginning. But, after doing daily changes for a few days, there will be almost no visible detritus, and that's when you can start to concentrate on the stuff that's fallen into the gravel. Get one of these:
This example shows what happens when nitrates are at 200, and is a bit conservative for a reading of that high... but they could be considerably higher, which is why I suggest such a slow approach. Better safe than sorry.