I had the same problem with one of mine, i was given the tank as a compleat system plus fish, the gravel was large uneven pea gravel, one of my goldfish got a particaly odd shaped bit stuck in its mouth I removed it and after sulking for a few hours the fish was fine.
I did however decided to replace the gravel with small stuff so that it wouldn't happen again.
Filtration The problem with filters is you have an ugly black box either taking up room in your tank or in the stand with a load of pipes and wires running all over the place. Which is probably why the ?designer? tanks all have crappy little filters? They don?t look good and so need to be hidden. But the filter media it?s self isn?t ugly and as all it needs to be is an inert material that has a high surface area it could be made to look good. So my plan was to have a built in filter taking up one end of the tank, but have it made out of clear material so you can see the media and it doesn?t become an oppressive presence in the tank. As it?s built in, there will be no need for any wires or pipes giving the tank a clear clutter free look which seems to be what the designer tanks are aiming for.
Lighting/tank lid LED?s are definitely the way forward with lighting, they are low energy and don?t produce a lot of heat. For my tank I would have the entire underside of the lid covered with LED?s to create one large light. I would also incorporate a couple of cameras in to the underside of the lid. The top of the lid would be a touch control display screen, from which you can control temperature lighting and water flow. This screen would show the footage from the cameras to create the illusion of a glass top.
Other equipment This would be a complete just a fish kind of deal so inside the stand I would include: ? 2 buckets ? A siphon ? A water test kit ? A net ? A fold up quarantine/hospital tank
The fold up hospital/quarantine tank The thinking behind this is that we all need a tank for treating sick fish or quarantine new stock, but it doesn?t have to be running all the time. So you will have to find storage space for a large bulky possible fragile thing that you may only use once or twice a year. I am basing this design on the fold up cage we have for taking the dog camping. It would consist of a flexible plastic tank that is supported on a rigid frame that can be taken apart to allow it to fold flat. It would have a mesh section at the top which can be unzipped to allow access to the fish. It would come with a small pump, which would have a media compartment which would be kept in the main tanks filter until it was needed.
I am also moving my goldfish out to a pond this year, I am in the process of building it now and plan to put the fish out once the night time temps pick up a bit, I am thinking may/june the risk of frost should have passed by then and they will have all summer to get used to the pond and for the filter to become establshed before they have to face winter.